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A 20-minute shower will usually use about 30 to 50 gallons of water, depending on the showerhead’s flow rate.
A showerhead that flows at 2.5 gallons per minute (gpm) uses about 50 gallons during a 20-minute shower. A 2.0 gpm showerhead uses about 40 gallons. EPA WaterSense-labeled showerheads are designed to use no more than 2.0 gpm, while the EPA describes a standard showerhead as using 2.5 gpm.
How to Calculate Water Used During a 20-Minute Shower
The calculation is simple:
Shower time × showerhead flow rate = water used
For a 20-minute shower:
20 minutes × gallons per minute = total gallons
Here are some common examples:
| Showerhead flow rate | Water used in 20 minutes |
|---|---|
| 1.5 gpm | 30 gallons |
| 1.8 gpm | 36 gallons |
| 2.0 gpm | 40 gallons |
| 2.5 gpm | 50 gallons |
For metric measurements, a 2.0 gpm showerhead is about 7.6 liters per minute. A 20-minute shower at that rate would use about 151 liters. EPA's WaterSense specification sets its maximum showerhead flow rate at 2.0 gpm, or 7.6 liters per minute.
How Much Water Does a Standard 20-Minute Shower Use?
Using the EPA's 2.5 gpm figure for a standard showerhead:
20 × 2.5 = 50 gallons
So a 20-minute shower with a standard 2.5 gpm showerhead can use about 50 gallons of water.
Actual use can be lower. Water pressure, the showerhead design, mineral buildup, and the position of the control valve can all change the real flow.
Also count only the time that water is actually running. If the shower is turned off while soaping or shampooing, total water use will be lower.
How Much Does a Water-Saving Showerhead Use?
A WaterSense-labeled showerhead has a maximum rated flow of 2.0 gpm.
At that rate:
20 × 2.0 = 40 gallons
That means a 20-minute shower would use up to about 40 gallons at the rated flow.
A lower-flow 1.5 gpm showerhead would use:
20 × 1.5 = 30 gallons
That is 20 gallons less than a 2.5 gpm showerhead over the same 20-minute shower.
How to Measure Your Shower's Actual Water Use
The number printed on the showerhead tells you its rated flow, but measuring the actual flow can give you a better estimate.
Use a container large enough to catch the shower water safely.
- Turn the shower on at the setting you normally use.
- Catch the water for 10 seconds.
- Measure how much water you collected.
- Multiply that amount by 6 to find the approximate flow per minute.
- Multiply the flow rate by 20 for a 20-minute shower.
For example, suppose you collect 0.33 gallon in 10 seconds.
Multiply by 6:
0.33 × 6 = about 2 gallons per minute
Then:
2 × 20 = about 40 gallons
You can use the same method with liters. If you collect 1.25 liters in 10 seconds, multiply by 6 to get about 7.5 liters per minute.
Study water use during a ten-minute shower to compare overflow capacity and ventilation for long-term storage.
Avoid trying to hold a small container under a strong shower spray if doing so could make the shower floor slippery.
Why Shower Water Use Matters for Rainwater Storage
Shower demand becomes especially important when planning a cistern or other stored-water system.
A single 20-minute shower could use 30 to 50 gallons. Two such showers per day could therefore require roughly 60 to 100 gallons per day just for showering.
For example, if two people each take one 20-minute shower using a 2.0 gpm showerhead:
2 people × 40 gallons = 80 gallons per day
Over one week:
80 × 7 = 560 gallons
That does not include toilets, sinks, laundry, irrigation, cleaning, or other uses.
This is why a rainwater storage system should be sized around actual water demand as well as roof collection potential. A large tank does not guarantee that rainfall will refill it fast enough to support regular shower use.
Using Rainwater for Showers
Using collected rainwater for bathing is more complex than using it for garden irrigation.
Roof runoff can contain dirt, bird droppings, microorganisms, roofing debris, and other contaminants. A screen or simple sediment filter does not automatically make rainwater suitable for showering.
Indoor rainwater systems may need suitable collection surfaces, debris removal, first-flush management, filtration, treatment, protected storage, plumbing controls, and ongoing maintenance. A first flush device diverts some of the first roof runoff so that accumulated debris does not go directly into the tank.
Requirements for indoor rainwater use vary by location. Follow current local plumbing and public-health requirements. If rainwater will supply showers or other household fixtures, system design and water treatment may be a job for a qualified professional rather than a simple rain-barrel project.
How Shower Length Changes Water Use
Reducing shower time can have as much effect as changing the showerhead.
With a 2.0 gpm showerhead:
| Shower length | Approximate water use |
|---|---|
| 5 minutes | 10 gallons |
| 10 minutes | 20 gallons |
| 15 minutes | 30 gallons |
| 20 minutes | 40 gallons |
| 30 minutes | 60 gallons |
The relationship is direct. Twice the shower time means roughly twice the water use if the flow stays the same.
Frequently Asked Questions
How many gallons does a 20-minute shower use?
A 20-minute shower generally uses about 30 to 50 gallons, depending on the showerhead's flow rate. A 2.0 gpm showerhead uses about 40 gallons, while a 2.5 gpm showerhead uses about 50 gallons.
Is a 20-minute shower a lot of water?
It can be. At 2.0 gpm, a 20-minute shower uses about 40 gallons. Multiple long showers can therefore make up a large part of daily household water demand.
How much water does a 10-minute shower use?
Multiply the flow rate by 10. A 2.0 gpm showerhead uses about 20 gallons in 10 minutes. A 2.5 gpm showerhead uses about 25 gallons.
How much water does a 15-minute shower use?
A 15-minute shower uses about 30 gallons with a 2.0 gpm showerhead or about 37.5 gallons with a 2.5 gpm showerhead.
How can I find my showerhead's flow rate?
Check the showerhead for a marked flow rate, usually given in gallons per minute. You can also measure the water collected over a known period and calculate the flow yourself.
Does low water pressure reduce shower water use?
It can. A showerhead may deliver less than its rated flow when the available pressure is low. Measuring the actual flow gives a better estimate than relying only on the rated maximum.
How much shower water should I include when sizing a rainwater tank?
Estimate the number of showers per day, average shower length, and actual showerhead flow rate. Then add other planned water uses. Tank sizing should also account for roof collection area, local rainfall patterns, storage losses, and dry periods.
