In short: a 10-gallon pressure tank, a 10 GPM pump, and 5 GPM of continuous demand produce a 4-minute cycle — 2 minutes on, 2 minutes off — which works out to 15 cycles per hour. That's in the marginal range; a bigger tank or lower demand would cut it down.
This is a planning aid using ideal on/off behavior. Real systems vary with pressure switch settings, tank pre-charge, and how demand actually fluctuates.
How to calculate pump cycle time
When water is drawn continuously at a rate below the pump's capacity, the pressure tank cycles even though someone is using water the whole time — the pump can't just turn on once and stay off, because demand keeps draining the tank between refills. Two phases repeat:
Worked example. Take the default case: a 10-gallon drawdown, a 10 GPM pump, and 5 GPM of demand. While the pump runs, it's adding 10 − 5 = 5 GPM net, so filling 10 gallons takes 10 ÷ 5 = 2 minutes on. While the pump is off, the tank drains at the full 5 GPM demand rate, so it takes 10 ÷ 5 = 2 minutes off. Total cycle: 4 minutes, or 15 cycles per hour.
Runtime per hour depends only on the demand-to-pump ratio. In this example the pump runs exactly half the time no matter how big the tank is — a bigger tank spreads that same 50% duty cycle over fewer, longer cycles, which is exactly why upsizing the tank is the standard fix for short-cycling without touching the pump or the demand.
Cycles per hour by tank drawdown
At a fixed 10 GPM pump and 5 GPM demand — only the tank size changes.
| Drawdown | Cycle time | Cycles/hour | Check |
|---|---|---|---|
| 2 gal · small | 0.80 min | 75.0 | Excessive |
| 6 gal · compact | 2.40 min | 25.0 | Marginal |
| 10 gal · standard (default) | 4.00 min | 15.0 | Marginal |
| 20 gal · large | 8.00 min | 7.5 | Good |
| 44 gal · extra-large | 17.60 min | 3.4 | Good |
Cycles per hour by demand rate
At a fixed 10-gallon drawdown and 10 GPM pump — only the demand rate changes.
| Demand | Cycle time | Cycles/hour | Pump runtime/hr |
|---|---|---|---|
| 1 GPM | 11.11 min | 5.4 | 6 min |
| 2 GPM · single fixture | 6.25 min | 9.6 | 12 min |
| 5 GPM (default) | 4.00 min | 15.0 | 30 min |
| 7 GPM | 4.76 min | 12.6 | 42 min |
| 9 GPM · garden hose | 11.11 min | 5.4 | 54 min |
Worked examples
Small tank, single fixture
45 cycles/hr
A 2-gallon drawdown tank with an 8 GPM pump against a 2 GPM fixture cycles every 1.33 minutes — 45 times an hour. Classic short-cycling from an undersized tank.
Standard tank, garden hose
6.75 cycles/hr
A 20-gallon tank, 12 GPM pump and a 9 GPM hose running cycles about every 8.9 minutes — comfortably under the usual 10-cycle-per-hour comfort threshold.
Large tank, irrigation zone
3.3 cycles/hr
A 44-gallon tank, 15 GPM pump and a 12 GPM irrigation zone cycles roughly every 18.3 minutes — well within a healthy range even under heavy, sustained demand.
What else affects the result
Pressure switch settings. A wider gap between cut-in and cut-out pressure increases the effective drawdown a tank delivers for the same physical tank — some of the "bigger tank" effect can be had by adjusting the switch, within the tank and pump's rated pressure range.
Intermittent vs. continuous demand. This calculator models one continuous draw. Real household demand is usually intermittent — a toilet fill, a shower, a dishwasher — so actual daily cycling is typically lower than this worst-case continuous-draw estimate, except during sustained use like irrigation or hose-filling.
Motor type and rating. Larger and submersible motors generally tolerate more starts per hour than smaller ones; always check your specific pump's rated maximum starts per hour rather than relying on a general rule of thumb for a borderline case.
Frequently asked questions
How do you calculate a well pump's cycle time?
Pump-on time equals the tank's drawdown volume divided by the pump's flow rate minus the demand rate: drawdown ÷ (Qpump − Qdemand). Pump-off time equals drawdown divided by the demand rate: drawdown ÷ Qdemand. Total cycle time is the sum of the two, and cycles per hour is 60 divided by the total cycle time in minutes.
What is short-cycling and why is it bad?
Short-cycling is when a pump turns on and off too frequently — often triggered by a pressure tank with too little drawdown for the demand it's serving. Frequent starts generate heat and mechanical stress each time, which shortens motor and pressure switch life well before their rated lifespan.
How many cycles per hour is too many for a well pump?
There's no single number that applies to every pump, but a widely used rule of thumb keeps cycling under about 10 starts per hour for comfortable long-term operation, with anything above roughly 20 to 30 starts per hour considered excessive for most residential pumps. Check your specific pump's manual for its rated maximum starts per hour.
How do I reduce how often my well pump cycles?
Install a larger pressure tank with more drawdown capacity, widen the gap between cut-in and cut-out pressure on the pressure switch (within the tank and pump's rated range), or reduce continuous demand — for example, running a garden hose or irrigation zone at a lower flow rate closer to the pump's output.
What happens if demand is higher than the pump's flow rate?
If demand meets or exceeds the pump's output, the tank can never refill faster than it's being drawn down, so pressure keeps falling and the pump runs continuously instead of cycling. That's a sign the pump is undersized for the demand and needs to be upsized.
What is drawdown volume on a pressure tank?
Drawdown volume is the amount of usable water a pressure tank delivers between the pump's cut-in pressure (when it turns on) and cut-out pressure (when it turns off). It's smaller than the tank's total volume because part of the tank stays filled with pressurized air or a compressed bladder. Manufacturers publish a rated drawdown figure for each tank model and pressure switch setting.
Sources & references
- Water Systems Council. Private well and pressure tank sizing guidance.
- Hydraulic Institute. Pump standards and motor duty-cycle guidance (ANSI/HI).
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)