Gallons Per Minute Calculator
Two ways to figure out flow rate. Bucket fill mode takes a real measurement: how much water you collected and how long it took. Flow rate conversion takes a number you already have in another unit and gives you GPM. Source units include liters per minute, cubic feet per second, gallons per hour, million gallons per day, liters per second, and cubic meters per hour.
Results
Estimates only. Bucket-fill measurements vary with how steady the tap pressure was during the fill and whether the bucket is rated to its labeled volume. Conversion math uses NIST SP 811 factors and assumes your source value is accurate. For sizing pumps, irrigation lines, or fire-suppression systems, confirm against a flow gauge or your supplier's spec sheet. TakeoffCalc isn't responsible for under- or over-sizing errors.
How to use this calculator
- 01Pick an input mode. Bucket fill works from a real measurement. Flow rate conversion works from a number you already have.
- 02For bucket fill, enter the volume of water you collected and how long it took. Pick seconds or minutes for the time unit.
- 03For flow rate conversion, enter your known flow rate and pick its unit from the dropdown.
- 04Read the gallons per minute (and liters per minute) at the top of the results panel.
- 05The rows below show the same flow rate in cubic feet per second, gallons per hour, cubic meters per hour, and liters per second.
Understanding the math
Bucket fill mode treats GPM as a rate. Take the volume you collected, divide by the time it took, then scale to a one-minute window. The seconds-to-minutes scale factor is 60 because a minute holds 60 seconds. The math is the same whether your bucket holds 5 gallons (19 liters) or 50 gallons (189 liters); the rate scales linearly.
GPM = (volume_gal / time_sec) × 60 GPM = volume_gal / time_min
Worked example. A 5-gallon bucket fills in 30 seconds. Volume is 5 gallons. Time is 30 seconds. (5 / 30) × 60 = 10 GPM. The same fill in 0.5 minutes gives 5 / 0.5 = 10 GPM. Both forms produce the same answer because they describe the same fill.
Flow rate conversion multiplies your source value by a fixed conversion factor. Each factor comes from NIST’s published unit definitions in NIST SP 811. The factors below convert from each source unit to GPM directly.
GPM = LPM × 0.264172 GPM = CFS × 448.831169 GPM = GPH × (1 / 60) GPM = MGD × 694.4444 GPM = L/s × 15.850323 GPM = m³/h × 4.402867
Worked example. A pump rated at 38 LPM converts to GPM. Source value is 38 liters per minute. The LPM-to-GPM factor is 0.264172. 38 × 0.264172 = 10.04 GPM. Round to display precision and the panel shows 10.0 GPM.
Common fixture flow rates
Typical flow rates for common fixtures. EPA WaterSense and the 1992 Energy Policy Act set most of the residential maximums; the rest come from manufacturer typical specs. Rates vary with water pressure, so treat these as planning numbers, not guarantees.
| Fixture | Flow rate (GPM) | Notes |
|---|---|---|
| Standard shower head | 2.5 GPM | 1992 federal maximum at 80 psi |
| WaterSense shower head | 2.0 GPM | EPA WaterSense voluntary spec |
| Kitchen faucet | 1.5 GPM | EPA WaterSense voluntary spec |
| Bathroom faucet | 1.2 GPM | EPA WaterSense maximum |
| Garden hose, 5/8 in at 50 psi | 9.0 GPM | Typical residential pressure |
| Garden hose, 3/4 in at 50 psi | 17.0 GPM | Larger diameter, same pressure |
| Toilet fill (standard) | 3.0 GPM | Fill cycle peak rate |
| Bathtub spout | 4.0 GPM | Typical residential spec |
| Washing machine fill | 3.0 GPM | Top-load cold and hot lines combined |
| Dishwasher fill | 1.5 GPM | Typical residential spec |
Frequently asked questions
How many gallons per minute does a shower use?
A standard shower head delivers 2.5 GPM (9.5 LPM), capped at that level by the 1992 Energy Policy Act at 80 psi. WaterSense-certified shower heads deliver 2.0 GPM (7.6 LPM) or less, which is the EPA's voluntary efficiency spec. Older shower heads from before 1994 may flow at 5 GPM or higher. Actual flow drops with lower water pressure, so a 2.5 GPM head running on 40 psi will deliver less than its rated maximum.
How to calculate gallons per minute
Two common ways. From a bucket fill, divide the volume you collected by how long it took, then scale to a one-minute window. A 5-gallon bucket filling in 30 seconds gives (5 / 30) × 60 = 10 GPM. From a known flow rate in another unit, multiply by the conversion factor. 38 LPM × 0.264172 = 10.04 GPM. The bucket method works at any tap, hose, or fill point. The conversion method works when a pump, fixture, or pipe spec already lists flow in non-GPM units.
What is gallons per minute
Gallons per minute (GPM) is a flow rate. It tells you how many US gallons of water move past a point in one minute. A kitchen faucet at 1.5 GPM means 1.5 gallons every 60 seconds. The unit is most common in US plumbing, irrigation, and fire protection. The metric equivalent is liters per minute (LPM), where 1 GPM equals about 3.79 LPM.
How many GPM do I need for a family of 5?
No single number applies because demand depends on what fixtures run at the same time. The honest approach is to add up your peak simultaneous load. A family of five during a busy morning might run two showers (5.0 GPM combined), one bathroom faucet (1.2 GPM), one toilet refilling (3.0 GPM), and a dishwasher (1.5 GPM) at once. That's 10.7 GPM at peak. The reference table above lists typical fixture flows you can sum for your own situation. A licensed plumber sizes the actual service line based on your fixture count, local pressure, and code requirements.
How much is 1 gallon per minute?
1 GPM equals 3.785 liters per minute, or about 0.0631 liters per second. Over an hour, 1 GPM totals 60 gallons (227 liters). Over a day, 1 GPM totals 1,440 gallons (5,451 liters). For comparison, a slow drip from a leaking faucet might be 0.05 GPM, while a fully open garden hose runs around 9 GPM at typical residential pressure.
Can I use this calculator with metric measurements?
Yes. The unit toggle accepts metric inputs. Bucket fill mode lets you enter volume in liters and time in seconds or minutes. Flow rate conversion accepts liters per minute, liters per second, and cubic meters per hour as source units. The output stays in GPM regardless of input system, since this is the gallons-per-minute calculator. A liters-per-minute calculator is on the build list and will give you LPM-native output when it ships.
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