Condensate pump sizing calculator
Size your mini-split condensate pump in seconds: tell us how far the drain water has to travel and we'll tell you the lift rating and flow you need — or whether a gravity drain saves you the pump entirely. Every number is computed from published sizing rules, cited below.
How condensate pump sizing works
A mini-split's indoor head pulls moisture out of the air, and that water has to go somewhere. When the drain line cannot fall continuously to its discharge point, a small condensate pump pushes it there instead. Sizing one comes down to two numbers: how high and far the water must travel, and how much water arrives per hour.
1. Total head: the height the pump must overcome
Pump manufacturer Beckett's sizing guide defines the requirement as total head = static head + friction head. Static head is the straight vertical distance from the pump outlet to the highest point of the drain line. Friction head accounts for drag in the horizontal tubing: Beckett's guidance is to assume at least 6 inches of head loss per 10 feet of level horizontal run. This calculator uses exactly those rules.
- Measure the vertical lift from the pump to the line's highest point.
- Measure the level run from that high point to the discharge.
- Required head = vertical lift + 0.5 ft for every 10 ft of horizontal run.
Worked example: a basement head that must lift condensate 12 ft to the ceiling and push it 20 ft across to a utility sink needs 12 + 1 = 13 ft of total head.
2. Flow: how much water the pump must move
Condensate volume depends on humidity and runtime, so pumps are sized with margin rather than to the average. This calculator budgets 0.5 gallons per hour per ton of system capacity — roughly 2 to 3 times typical condensing rates — so the pump stays ahead of the drain pan on the most humid days.
| Mini-split size | Sizing flow |
|---|---|
| 9,000 BTU | 0.4 GPH |
| 12,000 BTU | 0.5 GPH |
| 18,000 BTU | 0.8 GPH |
| 24,000 BTU | 1.0 GPH |
| 30,000 BTU | 1.3 GPH |
| 36,000 BTU | 1.5 GPH |
3. Read the flow curve, not the max-lift number
A pump's advertised max lift is where flow reaches zero. What matters is the rated head: the pump must still deliver your required gallons per hour at your total head. Beckett's guide makes the same point — choose a pump whose performance curve covers the duty point, then leave margin. If your required head lands within a few feet of a pump's max, step up.
When you don't need a pump at all
If the drain line can fall continuously from the head to the discharge — a steady slope of about 1/4 inch per foot — gravity does the work and a pump is one more thing that can fail. Pumps earn their keep in basements, below-grade rooms, and interior walls where the line must rise before it can fall. Our line-set install guide covers routing the drain alongside the refrigerant lines, and the pump itself tucks inside a line-set cover so nothing shows.
Source
Head formula and friction allowance: Beckett, How to Size and Choose a Condensate Pump. Flow sizing margin: industry practice of sizing 2 to 3 times above typical condensing rates, stated conservatively at 0.5 GPH per ton.
Common questions
What size condensate pump do I need for a mini-split?
Size by total head: the vertical lift from the pump to the highest point of the drain line, plus friction loss of about 6 inches per 10 feet of horizontal run. Pick a pump whose rated head at your required flow meets or beats that number. A 12 foot lift with a 20 foot horizontal run needs a pump rated for at least 13 feet.
Do I need a condensate pump for my mini-split?
Only if the drain line cannot fall continuously from the indoor head to the discharge point. A line with a steady downhill slope of about 1/4 inch per foot drains by gravity and needs no pump. Basements, interior walls, and below-grade rooms are the common cases that need one.
How much condensate does a mini-split produce?
It varies with humidity and runtime, but sizing at 0.5 gallons per hour per ton of capacity builds in a 2 to 3 times margin over typical condensing rates. A 12,000 BTU head sized this way needs a pump that can move 0.5 GPH at your required head.
What is the difference between max lift and rated head?
Max lift is the height at which the pump moves zero water. Actual flow falls as head rises, so size against the pump's flow curve: it must still deliver your required gallons per hour at your total head, not just reach it.
Can a condensate pump be hidden?
Yes. Mini-split condensate pumps are small enough to sit behind the indoor head or inside the line-set cover alongside the refrigerant lines, so the finished install shows no pump or tubing.
This calculator provides planning estimates based on Beckett's published condensate pump sizing method and a conservative flow allowance. It is not a substitute for your pump's performance curve or your unit's installation manual. Verify lift rating, flow at head, tubing size, and drain code requirements before purchasing or installing.