Why a sump pump is sized by flow at a head, not by horsepower
Every sump pump is sold with a horsepower on the box, and horsepower is close to useless on its own. What a pump actually delivers is a curve: gallons per minute plotted against the head it is working against. At zero head it moves a lot; at its shutoff head it moves nothing; in between it falls away steadily. Two pumps of the same nominal horsepower can differ by 40% in flow at 20 feet of head, because impeller design and motor speed differ.
So sizing has two halves, and this calculator does both. The first is the duty flow — how much water arrives, plus a margin. The second is the total dynamic head — how hard the discharge line makes the pump work, which is the vertical lift plus the friction the pipe and fittings add. Once you have a flow and a head, you have a point, and choosing a pump is a matter of finding a published curve that passes above it.
The measurement that matters most is the one you have to take yourself. Unplug the pump during sustained rain or spring melt, watch how fast the water climbs the pit, and time it. That rise rate, multiplied by the pit area, is the inflow. Doing it on a dry week gives you a number that means nothing, and the classic undersizing mistake is measuring on the wrong day.
Inflow, head and the friction the pipe adds
Inflow. The pit is a cylinder, so the volume arriving per minute is the cross-sectional area multiplied by the rise. A cubic foot holds 7.4805 US gallons, so Q(gpm) = area(ft²) × rise(ft/min) × 7.4805. An 18-inch pit has an area of π × 0.75² = 1.767 ft², which means each inch of rise is 1.767 × (1/12) × 7.4805 = 1.10 gallons.
Static lift is the vertical distance from the water level when the pump shuts off up to the highest point in the discharge line. It is head the pump must produce no matter how slowly it pumps, and it does not depend on flow.
Friction does depend on flow, steeply. This calculator uses the Hazen-Williams equation with C = 150 for PVC, in the customary form hf per 100 ft = 0.2083 × (100/C)1.852 × Q1.852 ÷ d4.8655, with Q in gpm and d the actual inside diameter in inches. Two features of that expression drive every practical decision: loss rises with flow to the power 1.852, so 50% more flow costs about 112% more friction; and it falls with diameter to the power 4.8655, so going from 1-1/2 inch to 2 inch pipe cuts friction by (2.067/1.610)4.8655 = 3.37 times at the same flow.
Fittings are converted to equivalent pipe length using the length-to-diameter ratios in Crane Technical Paper 410: about 30 diameters for a standard 90-degree elbow and about 100 diameters for a swing check valve. In 1-1/2 inch Schedule 40 pipe with a 1.610-inch bore, that is 4.03 feet per elbow and 13.42 feet for the check valve — which is why a check valve on a short run can be the largest single item of friction in the system.
The safety factor multiplies measured inflow. A pump sized exactly to the inflow runs continuously and has nothing left when the storm exceeds the one you measured. A factor of 1.5 is the common default; if your measurement was taken during a genuinely extreme event you might use less, and if it was taken during an ordinary shower you should use more.
Worked example: 30 gpm through a 1-1/2 inch line, 10 feet up
A basement pit takes 30 gpm during a spring thaw. The discharge is 1-1/2 inch Schedule 40 PVC, 30 feet of developed length, three 90-degree elbows, one swing check valve, and the outlet is 10 feet above the pump-off level.
- Apply the safety factor. 30 × 1.5 = 45 gpm required.
- Convert the fittings. The inside diameter is 1.610 in. Elbows: 30 × 1.610 ÷ 12 = 4.025 ft each, so 3 × 4.025 = 12.075 ft. Check valve: 100 × 1.610 ÷ 12 = 13.417 ft.
- Total equivalent length. 30 + 12.075 + 13.417 = 55.49 ft.
- Friction per 100 ft at 45 gpm. (100/150)1.852 = 0.47194 and 451.852 = 1152.4, while 1.6104.8655 = 10.145. So hf = 0.2083 × 0.47194 × 1152.4 ÷ 10.145 = 11.169 ft per 100 ft.
- Friction over this run. 11.169 × 55.49 ÷ 100 = 6.20 ft.
- Total dynamic head. 10 + 6.20 = 16.20 ft.
- Check the velocity. V = 0.4085 × 45 ÷ 1.610² = 7.09 ft/s, which is brisk but acceptable for an intermittent discharge.
So the pump you need is one whose published curve shows at least 45 gpm at 16.2 feet of head. Notice what the friction is doing: it is 6.20 ÷ 16.20 = 38% of the total head, and almost half of it comes from the check valve and elbows rather than from the straight pipe. Swap to 2-inch Schedule 40 and the friction at the same 45 gpm falls to 3.31 ft per 100 ft over a 62.73 ft equivalent length, or 2.08 ft — cutting total head from 16.20 ft to 12.08 ft, a 25% reduction, for the price of a few fittings.
Reading the duty point and the cycling interval
Match the pump to the point, not to the horsepower. Take the required gpm and the total dynamic head to the manufacturer's curve or table and confirm the pump delivers that flow at that head with margin. The horsepower band this calculator reports is a screening rule to narrow the shortlist — it is not a substitute for the curve, and pumps within a band vary widely.
The refill interval tells you about wear, not capacity. It is the pump-down volume divided by the inflow: how long the pit takes to fill from the switch-off level back to the switch-on level. Short intervals mean frequent starts, and motor and switch wear track starts far more closely than they track run hours. If the interval falls under a minute at your design inflow, the fixes are a larger pit or a greater vertical separation between the on and off levels — not a bigger pump, which makes cycling worse rather than better because it empties the pit faster.
Watch how much of the head is friction. Static lift is fixed by the building; friction is a design choice. When friction exceeds roughly a third of the total head, upsizing the discharge pipe usually buys more capacity per dollar than upsizing the pump, and it does so without adding starts.
Keep the discharge velocity moderate. Around 5 to 8 ft/s is comfortable for an intermittent line. Above 10 ft/s the friction term is climbing hard, and the line becomes noisy through the house. Below about 2 ft/s in a long horizontal run, sediment carried out of the pit can settle.
Back-up capacity is a separate calculation. A battery back-up pump is usually sized to the same inflow but delivers far less head per amp-hour, and its run time is limited by the battery. Size it against the inflow you must survive during an outage, which may be lower than the design storm figure.
Friction loss per 100 feet of Schedule 40 PVC discharge
| Flow (gpm) | 1-1/4 in (1.380 in ID) | 1-1/2 in (1.610 in ID) | 2 in (2.067 in ID) |
|---|---|---|---|
| 10 | 1.46 ft | 0.69 ft | 0.20 ft |
| 20 | 5.27 ft | 2.49 ft | 0.74 ft |
| 30 | 11.16 ft | 5.27 ft | 1.56 ft |
| 40 | 19.01 ft | 8.98 ft | 2.66 ft |
| 50 | 28.74 ft | 13.58 ft | 4.03 ft |
| 60 | 40.28 ft | 19.03 ft | 5.64 ft |
| 80 | 68.63 ft | 32.42 ft | 9.61 ft |
Equivalent lengths for fittings at 30 diameters per 90-degree elbow and 100 diameters per swing check valve: 3.45 / 11.50 ft in 1-1/4 in, 4.03 / 13.42 ft in 1-1/2 in, and 5.17 / 17.23 ft in 2 in pipe.
Screening bands for residential sump pumps
| Band | Duty flow up to | Total dynamic head up to |
|---|---|---|
| 1/3 hp | 30 gpm | 12 ft |
| 1/2 hp | 45 gpm | 18 ft |
| 3/4 hp | 60 gpm | 24 ft |
| 1 hp | 85 gpm | 30 ft |
| Above these | Select from a published curve, or split the duty between two pumps | |
This is a screening rule for narrowing a shortlist, not a performance specification. Pumps of the same nominal horsepower differ substantially, and the curve is the only authority.
Mistakes that undersize or wreck a sump pump
- Measuring the inflow on a dry week. The rise rate you need is the one during sustained rain or the spring thaw. Anything else understates the duty, often by an order of magnitude.
- Ignoring the check valve. In 1-1/2 inch pipe a swing check adds the resistance of about 13 feet of straight pipe. On a 20-foot run that is 40% more friction than the pipe itself.
- Using nominal pipe size in the friction formula. Schedule 40 bores are larger than the trade size: 1.610 in for 1-1/2 in pipe. Since loss goes as d to the power 4.87, using 1.5 instead of 1.610 overstates friction by about 40%.
- Fixing short cycling by fitting a bigger pump. A bigger pump empties the pump-down volume faster and starts just as often, since starts are set by inflow and pump-down volume. Increase the pit diameter or the switch separation instead.
- Discharging too close to the foundation. Water returned to the same soil comes straight back to the pit. The discharge belongs beyond the backfill zone and graded away, or into a storm system where that is permitted.
- Reading a pump's headline gph at zero head. Catalogue flow figures are often quoted at 0 or 10 feet. At your actual head the number is lower, sometimes much lower. Read the row that matches your head.
Where sump sizing meets the rest of the drainage design
A sump pump is the last resort in a drainage chain, and it works best when the links ahead of it are doing their share. Roof water should be leaving through correctly sized downspouts and discharging clear of the foundation; surface water should be graded away; the footing drain should be intact and flowing to the pit rather than into the soil. A pit that fills at 30 gpm in ordinary rain is usually reporting a problem upstream rather than a pump that is too small.
The head side of the calculation is the same total-dynamic-head arithmetic used for any pump, and if you want to work it in more detail — separating minor losses fitting by fitting or handling a longer, more complex line — use the total dynamic head calculator together with the K-factor and equivalent length tools. The friction model here is Hazen-Williams, which is the waterworks convention for cold water in the turbulent range and is what pump manufacturers publish against.
Where the storm load rather than groundwater is driving the pit, estimating the arriving flow from catchment area and rainfall intensity through the rational method gives you a design inflow independent of what you can measure on a given day, and it is worth comparing the two. And where the discharge runs to a gravity line, the receiving pipe has to take the peak too: check it with the Manning calculation at the grade you have.
Two last practical points. Codes in many jurisdictions prohibit discharging a sump into the sanitary sewer, because clean groundwater sent to a treatment plant displaces capacity that sewage needs. And a pump that is correctly sized still fails when the power does; if the basement is finished, the back-up system deserves the same arithmetic as the primary.
Key terms
- Total dynamic head (TDH)
- The total resistance a pump works against, in feet of water: the static lift plus the friction loss in the pipe and fittings. It is the horizontal axis you read a pump curve against.
- Static lift
- The vertical distance from the water surface at pump-off to the highest point in the discharge. It does not vary with flow, unlike friction.
- Equivalent length
- A way of accounting for fittings by expressing each as the length of straight pipe that would cause the same loss, typically 30 pipe diameters for a 90-degree elbow and 100 for a swing check valve.
- Pump-down volume
- The water removed between the switch-on and switch-off levels. Divided by inflow, it gives the interval between pump starts, which is what governs switch and motor wear.