SettingsSettings for this calculationUS
The total horizontal run of the gravity condensate drain line.
Measure the full horizontal length of the drain line from the equipment's drain pan connection to its termination or trap point.
The vertical drop per horizontal foot of drain line.
1/8 inch per foot (0.125) is the commonly-referenced minimum for gravity-drained HVAC condensate lines — some jurisdictions or manufacturers may require a steeper slope.
Minimum required drop
2.437 in
1/8 in per ft (10 mm per metre) is the commonly-referenced minimum slope for HVAC condensate drains — always verify against the applicable mechanical code edition and the specific equipment manufacturer's installation instructions, which occasionally specify a different minimum.
- Drain line length
- 19.5 ft
They open the calculator with your figures already in it
HVAC Condensate Drain Pipe Slope Calculator: 2.44 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Minimum condensate drain pipe slope of 1/8 inch per foot (≈1% grade) is the standard requirement referenced across the International Mechanical Code and manufacturer installation instructions for gravity-drained HVAC condensate lines, ensuring positive drainage to the termination point without standing water or biological growth.
Inputs used
- Condensate Drain Line Length
- 19.5 ft
- Slope (in per ft, 1/8 in/ft = 0.125 is the Common Minimum)
- 0.13
Intermediate steps
- Drain line length
- 19.5 ft
Confidence note: 1/8 in per ft (10 mm per metre) is the commonly-referenced minimum slope for HVAC condensate drains — always verify against the applicable mechanical code edition and the specific equipment manufacturer's installation instructions, which occasionally specify a different minimum.
What this calculation does not cover
- The fall this returns is horizontal; the number that most often fails is vertical. A drain leaving the negative-pressure side of a draw-through unit needs a trap deep enough to beat the cabinet's static — the usual rule sets the inlet leg at the fan's negative static pressure plus a margin, with a shallower outlet leg — or the fan holds the water in the pan and it overflows past a perfectly sloped line.
- The drop has to physically exist. Stack up the trap depth, the pipe diameter and the total fall computed above, and the whole lot has to fit between the drain pan connection and whatever sits beneath it — which on a ceiling-hung fan coil or a horizontal attic unit frequently it does not. That is the moment a condensate pump enters the job, and it is far cheaper to find before the unit is hung than after.
- Slope alone does not keep a line clear. Drain size follows the equipment's capacity, with three-quarter inch the usual minimum and larger lines above roughly 20 tons (18 tonnes), and the code also wants cleanout access at the trap and at changes of direction — a correctly sloped half-inch line still blocks with biofilm. Where a blockage would cause damage, a secondary drain pan or an overflow switch is required as well, and neither follows from the drop above.
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Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Minimum condensate drain pipe slope of 1/8 inch per foot (≈1% grade) is the standard requirement referenced across the International Mechanical Code and manufacturer installation instructions for gravity-drained HVAC condensate lines, ensuring positive drainage to the termination point without standing water or biological growth.
Which documents these citations point at
- International Mechanical Code (United States)Mechanical systems other than fuel gas — ventilation, exhaust, ducts and equipment.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
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Tools and safety for this job
To cut and join plastic pipe. Generic types, no brands, no prices.
Protection this work requires
- 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.
Cut and join plastic pipe: Push-fit and solvent-weld plumbing is within reach of a competent DIYer. The regulated parts of the system — gas, unvented hot water, and anything notifiable — are not, whatever tools are to hand.
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Essential
Plastic pipe shears
Tape measure
Recommended
Deburring tool and wire wool
what each plumbing tool is for, and the spec that decides which to buy where one does.