Why mastic instead of duct tape?

Despite the name, standard cloth-backed 'duct tape' dries out and fails at duct joints within a few years — mastic (a brushable sealant, often reinforced with mesh tape on wider gaps) is the durable, code-recognized standard for sealing duct seams.

How much energy loss does duct leakage typically cause?

Leaky ducts in unconditioned spaces (attics, crawlspaces) are a commonly cited significant source of forced-air HVAC system energy loss — sealing them is one of the higher-value, lower-cost efficiency improvements for ducted systems.

Do I need to seal ducts inside conditioned space too?

Leaks inside conditioned space don't waste energy the same way (the air stays inside the house), but sealing them still improves airflow balance and delivery to each room, which affects comfort even if it's not the top energy-efficiency priority.

How do I work out the seam length to enter?

It is a length of joint, not of duct. Every connection between two lengths is sealed the whole way round, so each one adds the distance round the duct at that joint; a seam running along a section adds that section's length; and each takeoff, boot or collar adds the distance round its own opening. A run built from short pieces with many connections can have more seam than it has duct, and a small duct adds less at each joint than a large one.

How is duct sealant sold in the UK and North America?

In North America duct mastic is usually a thick water-based compound brushed or trowelled on, sold in pails by the gallon and in gun-applied tubes, with coverage on the label in linear feet. In the UK 'mastic' is also everyday shorthand for any gun-applied sealant, silicone included, which is not a duct sealant, so check the product is made for ductwork. UK sheet-metal ductwork, like commercial ductwork in North America, is sealed as it is assembled —sealant in the seams and socket joints, gaskets on flanged joints — and circular ductwork with fittings that carry a factory-fitted rubber seal is made to seal without mastic at those joints, so leave them out of the length. A US quart is 0.946 litres, so the result is a little under the same number of litres.

Should the seams be sealed before the duct is insulated or boxed in?

Yes. Once a seam is under external insulation, behind a boxing-in or above a closed ceiling, it can no longer be reached, inspected or resealed, so seal first and let the mastic dry to the point the container states before anything covers it. Where a duct leakage test is required, testing after sealing but before covering lets a failing joint be found and put right while it is still exposed.

Why is the answer rounded up to whole quarts?

Because the count is of whole quarts, a job needing a little over one quart shows two; where the product is sold in gallon pails, round once more to what the shop sells. The breakdown line is the seam length with the brushing waste already added but before any rounding. Dividing that line once by the coverage of the container you choose, in the same length unit, and rounding up once avoids rounding up twice — first to quarts here, then again to containers.

Insulation & Efficiency

Duct Sealing Mastic Calculator

Estimate how much duct mastic sealant you need to seal your ductwork's seams and joints.

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

Who is doing the work?

The standard allowance most suppliers and estimating guides assume for ordinary work.

Total Seam Length

The combined length of all duct joints and seams to seal.

Add up the length of every duct joint, elbow connection, and register boot seam in the accessible ductwork you're sealing.

Brushing Waste (%)

Mastic left in the tub, on the brush and on the glove.

A thick sealant loads a brush heavily, and the last of a tub is hard to use cleanly. Overhead seams in a tight crawlspace or attic drip and waste more than seams at bench height, so raise the figure where most of the work is above your head.

Duct mastic needed

2 quarts

Medium confidence

Coverage varies with how thickly the mastic is applied and how wide the seams are — check your specific product's coverage rating.

Seam length (with waste): 53.9 linear ft

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

Show calculation logic

How this was calculated

Formula source(s)

Coverage derived from the applied geometry rather than quoted: a US quart is 57.75 in³, and a 2 in wide band at the ~1/16 in wet thickness duct mastic is brushed at gives about 38 linear ft per quart. A 3 in band at the same thickness gives about 26 ft
The widely published '100 linear ft' coverage figure for duct mastic is per GALLON, not per quart — four quarts — which is about 26 ft per quart and consistent with a 3 in band

Inputs used

Total Seam Length: 49 ft
Brushing Waste (%): 10

Intermediate steps

Seam length (with waste): 53.9 linear ft

Final result2 quarts

Confidence note: Coverage varies with how thickly the mastic is applied and how wide the seams are — check your specific product's coverage rating.

What this calculation does not cover

The whole answer rests on one coverage rate, worked from a 2 in band at about 1/16 in wet — roughly 38 linear ft per quart. There is no input for band width or thickness, and those two change the material per linear foot several times over: a 3 in band at the same thickness is about 26 ft per quart, and a heavy 1/8 in coat halves it again. Check the coverage printed on the tub you are buying and scale the result.
Published coverage figures for duct mastic are usually quoted per GALLON, and this result is in QUARTS — four to a gallon. A '100 linear ft' figure read off a datasheet and applied to a quart tub over-states the coverage fourfold, which is the error this calculator itself shipped with until 2026-09-02.
Only the seam length you type in is quantified — nothing here identifies which seams to measure. Register boots, plenum takeoff collars, the air-handler cabinet joints, flex-to-collar connections and the entire return side each leak, and any duct buried in a wall, floor cavity or sealed chase cannot be reached at all. Ductwork you cannot get to is not in the number and is not in the seal.
Nothing but the mastic itself is counted. Reinforcing mesh or fabric tape for gaps too wide for mastic alone, brushes, disposable gloves, and the time and material to clean dust and oil off galvanised flange before anything sticks are all separate. The result is also given in US quarts, which does not map onto the litre tubs sold outside North America.
A material quantity is not a leakage measurement. This does not tell you how leaky the system is now, how much it will improve, or what a duct leakage test will read. Where the adopted code requires a duct leakage test, the pass is decided by the measured result on the finished system, not by how much sealant was bought or applied.
Scope is HVAC supply and return air distribution. Duct mastic of this kind is not a sealant for flue pipe, vent connectors or anything else carrying combustion gases, and it is not what a grease-laden kitchen exhaust duct requires — that work needs continuous liquid-tight seams, not a brushed sealant. Do not use this quantity to plan sealing on either.

Estimated cost — your price

Price per quart ($)

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.

Schematic, drawn to the proportions you entered — not to scale on screen.

Part of bigger jobs

This trade is one line of several job takeoffs. Run the whole job and every other trade comes back with it, off the same measurements.

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 2026-09-17 · v1.1.0

Regulatory standards & verification citations2

Coverage derived from the applied geometry rather than quoted: a US quart is 57.75 in³, and a 2 in wide band at the ~1/16 in wet thickness duct mastic is brushed at gives about 38 linear ft per quart. A 3 in band at the same thickness gives about 26 ft
The widely published '100 linear ft' coverage figure for duct mastic is per GALLON, not per quart — four quarts — which is about 26 ft per quart and consistent with a 3 in band

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.

Tools and safety for this job

To run the ductwork and its fittings. Generic types, no brands, no prices.

Protection this work requires

Mineral and glass wool irritate skin, eyes and airways: cover your arms, wear a P2/N95 respirator while cutting, and rinse in cold water — hot water drives fibres in.
Boards, blocks and bagged material cause most lasting back injuries on small sites: two people or a lifter for full sheets, and never a bag on one shoulder up a ladder.
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.
Knives cause more site injuries than any power tool: cut away from your body, change blades often, and wear cut-resistant gloves to EN 388 level C for repeated cutting.

Run the ductwork and its fittings: Running, supporting and sealing duct is within reach of a competent installer. The duct sizes, routes and fittings come from the ventilation or air-handling design, and this site never sizes a duct; where a duct passes through a fire-separating wall or floor, the damper or fire-stopping it needs is set by the fire strategy and is the fire-stopping row's, not this one's. The hanger, mastic and wrap pages measure the run they are given.

Show the 7 tools this job needs

Essential

Cordless drill/driver
Tape measure
Utility knife

Recommended

Brushes
Crawl boards
Metal snips
Step ladder

Also needed as materials: drill and driver bits, replacement blades.

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

Now that you have the number

These guides cover the work this quantity is for.

Still deciding? Duct Sealing vs Envelope Sealing — the factors that actually differ, with no invented prices.

How to calculate duct sealing mastic in 3 steps

Total Seam LengthThe combined length of all duct joints and seams to seal.
Brushing Waste (%)Mastic left in the tub, on the brush and on the glove.
Duct mastic neededThe tool computes the duct mastic needed from those figures and shows the formula, its sources, and a confidence rating alongside it.

Duct mastic needed by total seam length

Page defaults, not your figures above.

Total Seam Length	Duct mastic needed (quarts)
20 ft	1
40 ft	2
60 ft	2
80 ft	3

Frequently asked questions

Why mastic instead of duct tape?: Despite the name, standard cloth-backed 'duct tape' dries out and fails at duct joints within a few years — mastic (a brushable sealant, often reinforced with mesh tape on wider gaps) is the durable, code-recognized standard for sealing duct seams.
How much energy loss does duct leakage typically cause?: Leaky ducts in unconditioned spaces (attics, crawlspaces) are a commonly cited significant source of forced-air HVAC system energy loss — sealing them is one of the higher-value, lower-cost efficiency improvements for ducted systems.
Do I need to seal ducts inside conditioned space too?: Leaks inside conditioned space don't waste energy the same way (the air stays inside the house), but sealing them still improves airflow balance and delivery to each room, which affects comfort even if it's not the top energy-efficiency priority.
How do I work out the seam length to enter?: It is a length of joint, not of duct. Every connection between two lengths is sealed the whole way round, so each one adds the distance round the duct at that joint; a seam running along a section adds that section's length; and each takeoff, boot or collar adds the distance round its own opening. A run built from short pieces with many connections can have more seam than it has duct, and a small duct adds less at each joint than a large one.
How is duct sealant sold in the UK and North America?: In North America duct mastic is usually a thick water-based compound brushed or trowelled on, sold in pails by the gallon and in gun-applied tubes, with coverage on the label in linear feet. In the UK 'mastic' is also everyday shorthand for any gun-applied sealant, silicone included, which is not a duct sealant, so check the product is made for ductwork. UK sheet-metal ductwork, like commercial ductwork in North America, is sealed as it is assembled —sealant in the seams and socket joints, gaskets on flanged joints — and circular ductwork with fittings that carry a factory-fitted rubber seal is made to seal without mastic at those joints, so leave them out of the length. A US quart is 0.946 litres, so the result is a little under the same number of litres.
Should the seams be sealed before the duct is insulated or boxed in?: Yes. Once a seam is under external insulation, behind a boxing-in or above a closed ceiling, it can no longer be reached, inspected or resealed, so seal first and let the mastic dry to the point the container states before anything covers it. Where a duct leakage test is required, testing after sealing but before covering lets a failing joint be found and put right while it is still exposed.
Why is the answer rounded up to whole quarts?: Because the count is of whole quarts, a job needing a little over one quart shows two; where the product is sold in gallon pails, round once more to what the shop sells. The breakdown line is the seam length with the brushing waste already added but before any rounding. Dividing that line once by the coverage of the container you choose, in the same length unit, and rounding up once avoids rounding up twice — first to quarts here, then again to containers.

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.