Toilet tank float arm and float ball (C) Daniel FriedmanToilet Flush Volume
How much water is used during a toilet flush
How to determine how much water a toilet uses at each flush

  • TOILET FLUSH VOLUME - CONTENTS: How to calculate or measure the water volume used when a toilet is flushed. What are standard toilet flush volumes? How much water is used when a water-saving toilet is flushed?
  • POST a QUESTION or READ FAQs about the volume of water in a toilet flush
InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Flush volume of water used in toilets:

This article explains how to calculate or measure the water volume used when a toilet is flushed. We also give standard toilet flush volumes? How much water is used when a water-saving toilet is flushed?

This article series describes the different types and models of toilets: historical or old toilet types, wooden high wall-tank toilets, conventional reservoir tank toilets, low-flush toilets, water saving toilets, back-flush toilets, up-flush toilets, and even chemical toilets. Here we explain how to diagnose and repair problems with toilets, leaks, flushes, odors, noises, running and wasted water.

Green links show where you are. © Copyright 2017, All Rights Reserved.

Toilet Flush Volume Standards & Calculations

Standard Toilet Water Flush Volumes


When water-saving toilets were first introduced in the 1980s, they reduced water usage from 5 to 6 gallons per flush (gpf) to 3.5 with little effect on performance.

However, when Congress mandated in 1992 that manufacturers had two years to reduce water usage to 1.6 gpf, the engineering challenges were much greater.

Most early toilet models were essentially 3.5 gallon designs hastily modified with smaller tanks and narrower trapways intended to increase the flow rate. Most did not work well and required two and sometimes three flushes, negating much of the benefit to water conservation.

How to Calculate the Flush Volume of an Installed Toilet

Reader Question: 12/16/2014 Anonymous said:

Is there a way to quickly identify the gallons per flush of commercial toilets (flushometer style) without taking them apart or metering flow? I know most newer toilets list the rate on the back of the bowl, but I’m not sure all do and older models do not. So, how do you know if you have an older 3.5 gpf (or more) unit or a newer 1.6 that just isn’t marked?

Reply: how to calculate toilet flush volume


If the toilet has no manufacturer's brand or model number we're in the dark on what it's doing for flush quantity. Worse, depending on the toilet's age, its flush mechanisms may have been modified from original, so a factory spec may not apply anyway.

Flush volume also may vary from one flush to the next depending on the cistern or tank fill rate and flush frequency.

How to calculate toilet tank volume, flush volume, in-fill volume

For a reasonable approximation of the toilet flush quantity, measure the internal dimensions of the tank. If we're lucky the tank is roughly rectangular. You want to measure the tank width and breadth. But for depth, measure just from the tank bottom to the top of water after the cistern has re-filled and the fill valve has stopped operating.

Calculate the toilet cistern or tank volume in inches:

Width X Depth X Water_Height = TTWV Cu-In = Toilet Tank Water Volume in cubc inches.

If you are in the U.S., convert the cubic inches to gallons by dividing:

Tank Water Volume (U.S. Gallons) = TTWV Cu-In / 231

That's the tank "full" volume.

If the tank is not absolutely square you can make some approximate adjustments to width or depth or height to get reasonably close. If the toilet tank is an oval we'd need to use a different formula for volume, or we'd take a simpler approach as follows.

How to estimate the volume of non-rectangular toilet tanks

Turn off water supply to the toilet.

Note the normal "full" level of water height in the tank.

Flush the toilet and hold the flush lever until the toilet is fully empty.

Using a container of known size such as a measuring cup. Pour water into the toilet tank, counting cups, until the tank has been filled to its full-line. Calculate the tank water volume as follows:

Tank Water Volume (U.S. Gallons) = Number of measuring cups / 16

NEXT Flush the toilet and watch what happens.

IF the tank empties fully before the tank flapper valve closes then the "full" volume is also the "flush" volume.

Subtract volume left at end of flush

IF the toilet tank flush valve closes BEFORE the tank has emptied fully then you need to note the depth of water in the tank, calculate that volume and subtract it from the "full" volume to get the "flush" volume. Also with the in-flow during-flush water volume I already explained.

Add fill-during-flush water volume

Actually I'm lying a little. Because the toilet fill valve will start sending water into the tank as soon as the flush starts, the true flush volume is a bit more than the tank volume. You'd need to catch that fill water for an interval - say a minute - and measure the volume entering the tank and multiply that by time that the flush valve sent water into the tank during the flush - before the flapper closed. You'd add that volume to the "full" or "flush" volume if you wanted to be more accurate.


Continue reading at TOILET REPAIR GUIDE or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see TOILETS, INSPECT, INSTALL, REPAIR. This toilet buy, install or repair guide article series discusses the cause, diagnosis, and repair of toilet problems (water closet problems) such as a toilet that does not flush well, clogged toilets, slow-filling toilets, running toilets, loose wobbly toilets, and odors at leaky toilets.

Suggested citation for this web page

TOILET TYPES, CONTROLS, PARTS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher's Google+ Page by Daniel Friedman