Introduction
Pump sizing math is simple enough that the formula rarely causes the failure — the inputs do. After the calculation comes a pump that won’t prime, a system that short-cycles itself to death, or a well pump that runs dry every August. Nearly all of these trace back to a short list of repeatable errors, most of them made before the calculator is ever opened.
This guide catalogs the mistakes that most often corrupt a pump sizing job, why each one matters in physical terms, and how to fix it. For the correct procedure, pair this with the Pump Size Calculator and the estimating guide — this page is the “what not to do” companion to both.
The Big Six
1. Ignoring friction loss entirely
The most common error in residential pump sizing. Someone measures the well depth, converts the pressure setting, adds them up, and calls it TDH — skipping the 15–40 ft of friction hiding in the pipe run, elbows, check valves, and the sediment filter. On a 200-ft TDH system, omitted friction is a 10–20% undersize, which lands the duty point off the pump’s curve: weak pressure at peak demand, a pump running at its ragged edge. Fix: walk the pipe run, count fittings as equivalent lengths, and look up friction per 100 ft for your pipe size and flow — the formula guide has the tables.
2. Sizing from the resting water level instead of the pumping level
Wells draw down under load — the water level while pumping is routinely 10–30 ft lower than the resting level. Using the resting level understates static head by the full drawdown, and the error strikes exactly when demand is highest. Fix: get the pumping water level from the driller’s yield-test log, and if the log is lost, assume a conservative drawdown or have a drawdown test run. Never size a well pump on a static-level guess.
3. Oversizing “to be safe”
The intuitive mistake: if some HP is good, more HP must be safer. In pumps it isn’t. An oversized pump reaches cut-out pressure almost immediately, then short-cycles — starting and stopping dozens of times an hour. That hammers the pressure switch, water-hammers the pipes, overheats the motor windings (starting current is the hardest duty a motor sees), and inflates the electric bill. Fix: size to the calculated duty point and round up only to the next standard pump size. Margin belongs in honest inputs, not in extra horsepower.
4. Assuming optimistic efficiency
Plugging 80% efficiency into the formula because the brochure mentioned it somewhere. Small residential pumps run 50–65% at their best point; the 80% figure, if it exists, belongs to a large commercial pump or to the motor alone. Every point of phantom efficiency shrinks the calculated HP and pushes you toward an undersized pump. Fix: default to 60% unless the pump’s published curve states efficiency at your duty point.
5. Putting a surface pump on a deep suction
Buying a bigger jet pump to “pull harder” from a water level 40 ft down. Atmospheric pressure caps practical suction lift at about 25 ft (7.6 m) — beyond that, the pump cavitates: it can’t fill with water, vibrates, and destroys its impeller and seals. Horsepower is irrelevant here; it’s a physics boundary, not a sizing problem. Fix: water deeper than 25 ft means a submersible (or a deep-well jet with the ejector down the well), full stop.
6. Starving the pump with an undersized pressure tank
A new 1.5 HP pump connected to a 20-gallon tank with a failed bladder: the pump starts every time a tap opens and stops seconds later. The pump didn’t fail — the system design did. Short cycling from inadequate drawdown is the leading cause of premature motor and switch death. Fix: size tank drawdown for at least one minute of pump runtime per cycle, set precharge 2 psi below cut-in, and replace waterlogged tanks with the pump.
Subtler Errors Worth Knowing
Mixing units mid-calculation
Entering flow in GPM but head in meters, or converting psi to head with the wrong factor (2.31 ft per psi, not 2.3-ish from memory — close, but do it exactly). The calculator keeps units consistent for you; hand-checks should too. Metric/imperial cross-checks — 0.42 HP vs 310 W for the same duty — catch these instantly.
Sizing for average demand instead of peak
The pump must satisfy the worst realistic simultaneous load, not the daily average. A household averaging 150 gallons a day still needs 10–12 GPM for the Saturday-morning peak. Average flow is for energy estimates; peak flow is for sizing.
Forgetting the filter’s dirty pressure drop
Sediment filters are sized on clean pressure drop, but they live dirty. A filter rated 2 psi clean can run 5–8 psi loaded — that’s 12–18 ft of head that appears gradually over months, slowly degrading system performance until someone blames the pump. Size friction with the dirty rating.
Ignoring voltage drop on long wire runs
A correctly sized pump on undersized wire runs hot, weak, and short-lived. Deep wells need the wire sized for under 5% voltage drop at full-load current — check the run length, not just the breaker size. This mistake masquerades as a “bad pump” and gets the pump replaced instead of the wire.
No dry-run protection on a well of unknown yield
A pump that can out-pump the well will eventually run dry — during a drought, a heavy irrigation day, or when a neighbor’s new well interferes. Dry running destroys a water-lubricated pump in minutes. A low-water cutoff costs a fraction of a pump replacement and belongs on every well of uncertain yield.
How These Mistakes Compound
The nasty property of sizing errors is that they stack. Optimistic efficiency (mistake 4) plus omitted friction (mistake 1) plus resting-level static head (mistake 2) can easily understate required HP by 30–40% — landing two pump sizes below what’s needed. Conversely, “safety” oversizing (mistake 3) on top of already-padded inputs produces the short-cycling monster. The defense is procedural: measure each input honestly per the estimating guide, run the Pump Size Calculator, cross-check units, and confirm the duty point on a real pump curve. The checklist compresses all of this into a pre-purchase verification pass.
Related Guides
- Pro Tips — the positive companion: field practices that prevent every mistake above.
- Estimating Guide — the correct measuring procedure, step by step.
- Pump Size Formula Explained — the math, so you can verify rather than trust.
- Case Study — a hypothetical system sized correctly, end to end.
- Pump Size FAQ — quick answers on head, flow, and efficiency.
- DIY vs. Pro — knowing which mistakes are DIY-safe and which need a contractor.
Frequently Asked Questions
What’s the single most common pump sizing mistake?
Omitting friction loss from TDH. It’s invisible (unlike well depth), tedious to tally (counting fittings), and large enough (10–25% of TDH) to change the pump size. If your sizing skipped friction, redo it.
Is it better to oversize or undersize a pump?
Neither — size it correctly. But if forced to choose the direction of a small error: slight oversizing (one standard size up) with a proper pressure tank is more forgiving than undersizing, which simply fails to deliver. Gross oversizing, though, is worse than either.
Why does my new pump short-cycle?
Almost always the tank: waterlogged bladder, wrong precharge, or drawdown too small for the pump’s flow. Check precharge first (2 psi below cut-in, tank drained, pump off) — it’s a five-minute test that explains most cycling complaints.
Can I fix an undersized pump by raising the pressure switch?
No — raising cut-out pressure increases the head the pump works against, which reduces flow further along the pump curve. An undersized pump needs a bigger pump (or less head: bigger pipe, lower pressure setting), not a higher switch setting.
My pump runs but delivers no water. Is it sized wrong?
Probably not — that’s a priming, suction-leak, or dry-well symptom, not a sizing symptom. Check prime, suction-line air leaks, foot valve, and water level before questioning the size. Sizing problems show up as weak performance, not zero performance.
Get Your Number
Every mistake above is avoidable with the same discipline: honest inputs, counted fittings, pumping-level static head, realistic efficiency, and a pump curve check. Run your corrected numbers through the Pump Size Calculator, verify with the checklist, and you’ll have sized on evidence instead of optimism.
Disclaimer: This guide describes common planning and installation errors for educational purposes. It is not a substitute for professional system design. Well yield, water chemistry, and electrical conditions vary by site — consult a licensed well contractor or pump professional before selecting or installing equipment.
