Why does the page tell me the batch size rather than the total?

Because the total is not the number that gets a two-pack job wrong. Every other paint calculator answers how much to buy, and for single-pack paint that is the whole question — what is left in the tin at the end of the day goes back on the shelf. A two-pack starts curing the instant the parts meet, so the question becomes how much you dare combine at once, and the answer has nothing to do with the size of the job. It is set by how fast you can physically lay the material down and how long the mix stays usable. Mix the job's total in one go because it is convenient and you will be scraping most of it out of the bucket as a solid, which is both a wasted material cost and a chemical waste disposal problem rather than a rinse. The total still matters for ordering, and it is on the page — but the number to work to is the batch.

Why can I not add a little extra hardener to be safe?

Because a two-pack is not a paint that dries, it is two chemicals that react with each other in a fixed proportion, and there is no safety margin in either direction. The base and the hardener combine molecule for molecule; whatever is supplied in excess of that proportion has nothing to react with and simply stays in the film. Too little hardener and the coating never reaches full cure — it stays soft, remains sensitive to solvents, and can still be marked with a fingernail weeks later. Too much and the surplus makes the film brittle, and on some chemistries it migrates to the surface as a greasy bloom that defeats anything applied over it. The instinct that more catalyst means a harder, faster, better cure is imported from materials that work differently, and it is the most confident way to ruin a floor. Measure it, do not judge it.

Does it matter whether the ratio is by volume or by weight?

Yes, and the difference is large enough to ruin the coating. The two parts of a two-pack have different densities, so four volumes of base is not four weights of base — a 4:1 by-weight product mixed as 4:1 by volume gives a genuinely off-ratio mix. Data sheets state which basis they mean, and they do not always make it prominent. The practical consequences are that a by-weight ratio needs scales rather than a graduated bucket, and that you cannot convert between the two without knowing both densities, which are on the same data sheet but rarely to hand. What makes this error dangerous is that it leaves no immediate trace: the mixed coating looks right, applies normally, and then under-cures over the following weeks. If you have only a marked bucket and the sheet quotes weight, the answer is to get scales rather than to approximate.

Why does a big pot go off faster than a small one?

Because the reaction gives off heat, and heat accelerates the reaction. A large mass of mixed coating in a deep container holds that heat in — the material at the centre is insulated by the material around it — so the batch warms itself, cures faster, and generates heat faster still. The effect is sharp enough that the same product mixed in a five litre bucket and in a shallow tray behaves noticeably differently. It is also why a marginal batch can be rescued by pouring it out: decanting the mix into a wide roller tray immediately after combining spreads the heat, buys working time, and has the useful side effect of putting the material where you need it. The corollary is that a pot left standing in direct sun, or mixed in a hot van, can go off in a fraction of its quoted life, and the quoted life assumed neither.

Materials & Quantities

Two-Pack Paint and Coating Mixing Calculator — Pot Life and Batch Size

How much base and hardener to mix, and how much you dare mix at once — because a two-pack's limit is the pot life rather than the size of the job.

Answers as you typeNo submit button
Every formula citedNamed published standards
Calculated in your browserNo account, nothing uploaded

SettingsSettings for this calculationUS

Market

Imperial · sales tax

Mixed coating the job needs

The total volume of MIXED product for the whole job.

Mixed, not base — a 4:1 product needs four parts base and one part hardener to make five parts of coating, so the base you buy is less than the coating you spread. Take the total from the coverage rate on the data sheet and the area, remembering that a two-pack is usually specified by dry film thickness rather than by a spreading rate.

Parts base

The first number in the product's ratio — the 4 in 4:1.

Straight off the data sheet or the tin. Where the product is supplied in matched pre-measured units the ratio is already handled for you, and the right move is to mix whole units rather than to decant part of one.

Parts hardener or activator

The second number in the ratio — the 1 in 4:1.

This is the number that is never rounded generously. Adding extra hardener does not make a two-pack cure harder or faster; it leaves unreacted material in the film and makes the coating brittle.

Is the ratio by volume or by weight?

Data sheets quote one or the other, and they give different mixes.

The two bases disagree because base and hardener have different densities, and the disagreement is large enough to matter. Mixing a by-weight ratio as though it were by volume is a genuine error with no visible symptom until the coating fails to cure properly.

Pot life at the quoted temperature

Minutes of usable time after mixing, from the data sheet.

Pot life is not a cliff edge — the mix thickens throughout, and the last few minutes of it apply far worse than the first. Treat the quoted figure as the point by which the pot must be empty rather than the point at which you stop pouring.

Working temperature

The temperature on site while the coating is being applied.

The temperature of the MIX matters more than the air, and a full pot generates its own heat as it reacts — which is why a large batch gels faster than a small one and why a pot left standing in sun can go off in a fraction of its rated time. Enter the honest site temperature rather than the design one.

How fast you can apply it

Volume of mixed coating you can lay down in an hour.

Be honest and be pessimistic. This is the number that decides the batch size, and over-estimating it is how a pot gels in the tray. Include the time spent cutting in, moving equipment and dealing with interruptions, because the pot life is running through all of it.

Largest batch to mix at once

2.17 gal

Medium confidence

4 : 1 BY VOLUME — measure in a graduated container, and to that basis only. At 68 °F you have about 40 minutes of working time. Mix 2.2 gal at a time, 3 batches for the job. Never add extra hardener: excess stays unreacted in the film and makes the coating brittle.

Base in one batch: 1.73 gal
Hardener in one batch: 0.43 gal
Batches for the whole job: 3 batches
Working time at this temperature: 40 min
Base for the whole job: 4 gal
Hardener for the whole job: 1 gal

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

The mixing ratio and the pot life both come from the product's own technical data sheet, and the ratio is stated there as either BY VOLUME or BY WEIGHT. The two are not interchangeable because the parts differ in density, and this page applies the ratio to whichever basis you select
Pot life is quoted at a reference temperature, conventionally around 20 °C (68 °F), and shortens sharply as temperature rises — a widely used rule of thumb is that it roughly halves for every 10 °C above the quoted figure. That is an approximation for planning the batch size, not a property of any particular product
A two-pack cures by reaction between the two parts in fixed proportion. Excess of either remains unreacted in the film, so an off-ratio mix is a permanently defective coating rather than a slightly different one — under-catalysed stays soft, over-catalysed turns brittle

Inputs used

Mixed coating the job needs: 5 gal
Parts base: 4
Parts hardener or activator: 1
Is the ratio by volume or by weight?: By volume — measured in a graduated container
Pot life at the quoted temperature: 40
Working temperature: 68 °F
How fast you can apply it: 3.25 gal

Intermediate steps

Base in one batch: 1.73 gal
Hardener in one batch: 0.43 gal
Batches for the whole job: 3 batches
Working time at this temperature: 40 min
Base for the whole job: 4 gal
Hardener for the whole job: 1 gal

Final result2.17 gal

Confidence note: 4 : 1 BY VOLUME — measure in a graduated container, and to that basis only. At 68 °F you have about 40 minutes of working time. Mix 2.2 gal at a time, 3 batches for the job. Never add extra hardener: excess stays unreacted in the film and makes the coating brittle.

What this calculation does not cover

The ratio and the batch size here are arithmetic; the shade is not. A two-pack's colour comes from the tinted base you bought and nothing on this page predicts or shows it. Where the colour itself is in question, spray or brush a sample onto a board and judge it cured, because a catalysed finish rarely dries to the colour it goes on as.
The ratio and the pot life belong to YOUR product and must come from its data sheet. Both vary enormously between products that look identical in the tin, and nothing here is a substitute for reading the one in your hand.
The ratio applies to the basis you selected and to no other. A by-weight ratio mixed as though it were by volume — or the reverse — is a genuine error that shows no symptom until the coating fails to reach full cure, because the two parts have different densities.
The temperature adjustment is a rule of thumb for planning a batch size, not a property of any product. Real pot life depends on the chemistry, on the mass of the batch, and on the shape of the container, and a manufacturer's figure at a stated temperature governs over this arithmetic.
A big pot gels faster than a small one at the same temperature. The reaction generates heat and a deep container holds it in, so the same mix in a wide shallow tray lasts noticeably longer than in a tall bucket — which is one practical way to buy time when a batch is marginal.
Pot life is not a cliff edge. The mix thickens from the moment it is combined, and coating applied in the last minutes of its life flows and levels far worse than coating applied in the first. Plan for the pot to be EMPTY at the quoted time, not to stop pouring then.
Nothing here addresses induction time, surface preparation, overcoating windows or humidity limits, and any of those will ruin a correctly mixed coating. A two-pack is a system with a specification, and the mixing arithmetic is the easy part of it.
Two-pack polyurethanes are cured with ISOCYANATES, which are respiratory sensitisers: once someone is sensitised, later exposure at far lower levels can provoke a reaction, and the effect does not reverse. Spray application is the high-risk case because it puts the hardener into the air as droplets. The protection this needs is set by the product's safety data sheet and by the regulations where you are working, and neither is something a quantity calculator should be specifying — read the sheet before mixing, not after.

Estimated cost — your price

Price per gal ($)

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-15 · v1.0.0

Regulatory standards & verification citations3

The mixing ratio and the pot life both come from the product's own technical data sheet, and the ratio is stated there as either BY VOLUME or BY WEIGHT. The two are not interchangeable because the parts differ in density, and this page applies the ratio to whichever basis you select
Pot life is quoted at a reference temperature, conventionally around 20 °C (68 °F), and shortens sharply as temperature rises — a widely used rule of thumb is that it roughly halves for every 10 °C above the quoted figure. That is an approximation for planning the batch size, not a property of any particular product
A two-pack cures by reaction between the two parts in fixed proportion. Excess of either remains unreacted in the film, so an off-ratio mix is a permanently defective coating rather than a slightly different one — under-catalysed stays soft, over-catalysed turns brittle

Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Protection this work requires

Stop and check first. Paint under later coats in older housing can be lead-based, and sanding or heat-stripping it produces lead dust that is especially harmful to children and to anyone pregnant. Test before you disturb it — this is a job for a tested, controlled method, not a better mask.

Tools and safety for this job

To roll and cut in two coats and prepare a surface for paint. Generic types, no brands, no prices.

Joint-compound dust is very fine and travels: a P1/FFP1 mask is the minimum, and a damp-sponge finish avoids most of it.
A dust mask does nothing about solvent vapour: ventilate through, and use an organic-vapour (A1) cartridge respirator in an enclosed room.
Most fatal falls on small jobs are from under three metres: use a tower or a ladder tied at the top, and never work off the top two rungs.

Show the 11 tools this job needs

Essential

Brushes
Dust sheets and masking
Filling knife
Roller frame, sleeves and tray
Scraper

Recommended

Caulking gun
Extension pole
Pole sander
Random orbital sander
Step ladder

Optional

Dust extractor or vacuum with a fine filter

Also needed as materials: abrasive discs, caulk and sealant cartridges, masking tape, plastic sheeting, roller sleeves, sanding screens or paper.

what each painting tool is for, and the spec that decides which to buy where one does.

The data behind it: Production rates by operation

How to calculate two-pack paint and coating mixing — pot life and batch size in 8 steps

Mixed coating the job needsThe total volume of MIXED product for the whole job.
Parts baseThe first number in the product's ratio — the 4 in 4:1.
Parts hardener or activatorThe second number in the ratio — the 1 in 4:1.
Is the ratio by volume or by weight?Data sheets quote one or the other, and they give different mixes.
Pot life at the quoted temperatureMinutes of usable time after mixing, from the data sheet.
Working temperatureThe temperature on site while the coating is being applied.
How fast you can apply itVolume of mixed coating you can lay down in an hour.
Largest batch to mix at onceThe tool computes the largest batch to mix at once from those figures and shows the formula, its sources, and a confidence rating alongside it.

Frequently asked questions

Why does the page tell me the batch size rather than the total?: Because the total is not the number that gets a two-pack job wrong. Every other paint calculator answers how much to buy, and for single-pack paint that is the whole question — what is left in the tin at the end of the day goes back on the shelf. A two-pack starts curing the instant the parts meet, so the question becomes how much you dare combine at once, and the answer has nothing to do with the size of the job. It is set by how fast you can physically lay the material down and how long the mix stays usable. Mix the job's total in one go because it is convenient and you will be scraping most of it out of the bucket as a solid, which is both a wasted material cost and a chemical waste disposal problem rather than a rinse. The total still matters for ordering, and it is on the page — but the number to work to is the batch.
Why can I not add a little extra hardener to be safe?: Because a two-pack is not a paint that dries, it is two chemicals that react with each other in a fixed proportion, and there is no safety margin in either direction. The base and the hardener combine molecule for molecule; whatever is supplied in excess of that proportion has nothing to react with and simply stays in the film. Too little hardener and the coating never reaches full cure — it stays soft, remains sensitive to solvents, and can still be marked with a fingernail weeks later. Too much and the surplus makes the film brittle, and on some chemistries it migrates to the surface as a greasy bloom that defeats anything applied over it. The instinct that more catalyst means a harder, faster, better cure is imported from materials that work differently, and it is the most confident way to ruin a floor. Measure it, do not judge it.
Does it matter whether the ratio is by volume or by weight?: Yes, and the difference is large enough to ruin the coating. The two parts of a two-pack have different densities, so four volumes of base is not four weights of base — a 4:1 by-weight product mixed as 4:1 by volume gives a genuinely off-ratio mix. Data sheets state which basis they mean, and they do not always make it prominent. The practical consequences are that a by-weight ratio needs scales rather than a graduated bucket, and that you cannot convert between the two without knowing both densities, which are on the same data sheet but rarely to hand. What makes this error dangerous is that it leaves no immediate trace: the mixed coating looks right, applies normally, and then under-cures over the following weeks. If you have only a marked bucket and the sheet quotes weight, the answer is to get scales rather than to approximate.
Why does a big pot go off faster than a small one?: Because the reaction gives off heat, and heat accelerates the reaction. A large mass of mixed coating in a deep container holds that heat in — the material at the centre is insulated by the material around it — so the batch warms itself, cures faster, and generates heat faster still. The effect is sharp enough that the same product mixed in a five litre bucket and in a shallow tray behaves noticeably differently. It is also why a marginal batch can be rescued by pouring it out: decanting the mix into a wide roller tray immediately after combining spreads the heat, buys working time, and has the useful side effect of putting the material where you need it. The corollary is that a pot left standing in direct sun, or mixed in a hot van, can go off in a fraction of its quoted life, and the quoted life assumed neither.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.