What size sump pump do I need?

Two measurements and ten minutes give you a real answer. Almost everyone skips both and buys on horsepower instead.

Sump pump sizing has a reputation for being complicated, and it is not. There are two numbers: how high the pump has to push water, and how fast water arrives. Get both and the choice is obvious. Skip both — which is the norm — and you are buying on a horsepower figure that describes very little.

Step 1: measure the total lift

1. Step 1Measure the vertical heightFrom the pump's discharge connection up to the highest point the water reaches in the pipe, usually where it exits at the rim joist. In most basements this is 8 to 10 feet.
2. Step 2Add for the horizontal runAdd roughly one foot of equivalent head for every 10 feet of horizontal pipe. A 30-foot run outside therefore adds about 3 feet.
3. Step 3Add for fittingsEach 90-degree elbow adds a small amount of equivalent head. Several of them add up; count them and add a foot or two.
4. Step 4That total is your liftCompare pumps at THIS number, not at zero feet. It is usually between 10 and 14 feet once everything is counted.

Step 2: measure the inflow

Do this during steady rain, which is when the number means something. Let the pit fill to the point where the pump would normally switch on, then unplug the pump. Time how long the water takes to rise one inch, then plug the pump back in.

Those figures look small next to a pump's rated gallons per hour, and that is the point: a healthy pump should clear ordinary inflow easily. If your measured inflow is anywhere near the pump's flow at your lift, you are at the limit and need more capacity — or you need to find out why so much water is arriving.

Step 3: pick the pump

• Total lift under 10 ft, modest inflow: 1/3 HP. This is most houses.
• Lift 10 to 15 ft, or fast refill: 1/2 HP.
• Lift over 15 ft, or very large collection area: 3/4 HP, and check the discharge pipe can take the flow.
• Inflow still exceeding capacity: the problem is drainage, not the pump. A second pit or exterior work is the real answer.

#1Up to about 10 ft of lift, ordinary inflow

Zoeller M53 Mighty-Mate

The 1/3 HP answer for most houses, with published flow you can size against.

8.9/10★★★★★Our editorial score: 8.9 out of 10

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Specifications, as published by Zoeller

Type	Submersible, cast iron
Horsepower	3/10 HP (sold as 1/3 HP)
Electrical	115 V, 9.7 A, 60 Hz
Discharge	1-1/2" NPT vertical female
Switch	Integral vertical float, on at 7-1/4", off at 3"
Solids handling	1/2"
Shut-off head	19.25 ft
Flow, by lift	2,580 GPH at 0 ft · 2,040 GPH at 10 ft
Warranty	Three years

If the measurements put you in the ordinary range — and most houses are — this is where to start. Published flow at multiple lifts is what lets you check that rather than assume it.

What it gets right

• Flow published at several lifts
• Cast iron
• The most widely supported pump in the category

Where it falls short

• Marginal above 12 ft of lift
• Tethered float needs pit width

Specifications from Zoeller M53 (53-0001) specification sheet.

#2High lift, or a pit that refills fast

Zoeller 1075 (1/2 HP cast iron submersible)

Step up to 1/2 HP when the arithmetic says the smaller pump is at its limit.

8.7/10★★★★★Our editorial score: 8.7 out of 10

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Specifications, as published by Zoeller

Type	Submersible, cast iron
Horsepower	1/2 HP
Line	Zoeller at Home consumer range

Step up because the numbers say so. Oversizing on instinct produces a pump that empties the pit in seconds and starts again two minutes later, which wears the switch out.

What it gets right

• Holds useful flow past 12 ft
• Same installation as the 1/3 HP version
• Cast iron

Where it falls short

• Short-cycles in a small pit
• Higher current draw

Specifications from Zoeller at Home sump pump range.

If the pit itself turns out to be the constraint, what size sump pit covers that. If you are choosing between specific sizes, 1/3 HP vs 1/2 HP works through the comparison with numbers.