By Saturday Gear Co. Editor · Published October 2, 2026
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Why the washer's GPM rating is not the whole answer
A washer rated at 2.5 GPM does not automatically work with a hose that barely measures 2.5 GPM in an open bucket. The pump needs a reliable feed under the maker's specified inlet conditions. Requirements differ by brand and model: SIMPSON's general FAQ specifies at least 5 GPM at 20 PSI for its pressure washers, while Kärcher's U.S. residential FAQ calls for 4 GPM at 40 PSI. Those are manufacturer-specific examples, not one rule for every machine.
Before shopping by output, find the water-supply section in the manual for the precise model and revision you intend to buy. Check permitted water temperature and supply method as well as flow and pressure.
Measure your hose flow with a bucket
- Use the outdoor spigot and the full garden-hose run you actually plan to use, with the same fittings. Remove any spray nozzle from the hose end.
- Put the free end in a bucket with a known gallon mark. Turn the spigot fully on and time how long it takes to reach that mark. Keep the hose safely positioned so it cannot whip or spill.
- Calculate GPM = gallons collected × 60 ÷ seconds. For example, five gallons in 75 seconds is 4 GPM; five gallons in 60 seconds is 5 GPM.
- Repeat when other household water uses may run at the same time. A well pump, irrigation system or another faucet can change what the washer receives.
Kärcher describes a one-minute, five-gallon-bucket version of this test in its U.S. FAQ. The arithmetic above simply lets you use a different measured volume or time. This test checks open hose flow; it does not measure the pressure available at the washer inlet while water is moving. If the manual specifies an inlet-pressure minimum and you cannot verify it, consult the manufacturer or a qualified water-system professional.
Three buying outcomes
The supply clears the exact model's requirements
Continue comparing the machine's rated operating PSI and GPM against the jobs you actually do. Our pressure-washer sizing and product guide explains the cleaning tradeoffs and identifies a research-based current DEWALT option separately from the editor's older owned machine.
The hose flow falls short
First remove avoidable restrictions: uncoil the hose, clear kinks, inspect fittings and the inlet screen, and retest. A shorter, suitable-diameter hose may help, but it cannot fix an inadequate spigot or well supply. Choose a machine whose actual manual requirements fit the source only after confirming both flow and pressure.
You plan to feed from a tank or standing water
Do not assume a standard pressure washer can draw from a tank. Some machines explicitly support suction with a specified hose and filter; others require a positive-pressure feed. Follow the exact manual, especially for filtration, lift height and priming. A bucket that holds enough water is not proof that the pump can draw it.
Check the hose and the source together
| Check | Why it matters | What to do |
|---|---|---|
| Hose diameter and length | A narrow, long or kinked run can limit water reaching the pump. | Use the manual's permitted hose. SIMPSON recommends an unrolled 5/8- or 3/4-inch hose and the shortest practical run; Kärcher describes 5/8 inch or larger and 50 feet or less for its residential units. |
| Spigot and competing use | Supply may drop when irrigation, laundry or other taps run. | Test the intended spigot during realistic household use, especially on a well. |
| Clean inlet screen and purge | Debris or trapped air can mimic a weak supply. | With the machine off, inspect the inlet screen, flush the garden hose, reconnect, then purge air according to the manual before starting. |
| Water source and temperature | A cold-water pump or non-suction model has limits the output label does not show. | Use only a source and temperature the manual allows. |
If pressure drops during use
Stop and check the supply before assuming the washer needs a larger pump. SIMPSON's low-pressure troubleshooting starts with the inlet screen, trapped air and water source, then checks nozzles. Shut the machine down before disconnecting hoses or clearing screens. Do not run a pressure-washer pump without its specified water supply: SIMPSON's manual explains that water cools and lubricates pump parts.
For a long driveway, the next purchase question is whether an electric or gas setup offers the useful flow and ownership tradeoffs you want. If you already have a compatible washer, our surface-cleaner sizing guide covers attachment PSI, GPM and fittings. Check the water feed first in either case.
Sources checked October 2, 2026: Kärcher U.S. pressure-washer FAQ and bucket test · SIMPSON water-supply and hose FAQ · SIMPSON low-pressure troubleshooting · SIMPSON manual and pump-purging instructions. Follow the manual for your exact model.