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A runoff coefficient tells you what share of the rain falling on a catchment surface actually becomes usable runoff.
For rainwater harvesting, you can estimate it with this basic formula:
Runoff coefficient = collected runoff volume ÷ rainfall volume falling on the catchment
A coefficient of 0.85, for example, means about 85% of the rain falling on that surface became runoff. The rest was lost to wetting, evaporation, splashing, small leaks, first-flush diversion, or water left on the roof.
How to Calculate a Runoff Coefficient
You need three measurements:
- Catchment area
- Rainfall depth
- Runoff volume collected
First calculate how much rain fell on the catchment. Then compare that theoretical volume with the amount of runoff you actually collected.
Step 1: Measure the Catchment Area
For a roof, use the horizontal footprint that drains to the gutters you are measuring.
Do not normally use the sloped surface area of the roofing material. Rainfall depth is measured over a horizontal area, so the projected roof footprint is the useful measurement.
If only half of your roof drains to the tank, include only that half.
For a simple rectangular catchment:
Area = length × width
For example:
40 ft × 25 ft = 1,000 sq ft
Step 2: Calculate the Rainfall Volume
Multiply rainfall depth by catchment area, using compatible units.
In U.S. units, one inch of rain on one square foot equals about 0.623 gallons.
So:
Rainfall volume in gallons = roof area in sq ft × rainfall in inches × 0.623
For a 1,000-square-foot roof receiving 1 inch of rain:
1,000 × 1 × 0.623 = 623 gallons
That is the theoretical amount of water that fell on the catchment.
It does not mean you should expect 623 gallons to reach your tank.
Step 3: Measure the Actual Runoff
Next, determine how much water actually came off the catchment.
For a small test area, you may be able to collect the runoff in a container and measure it directly.
For a tank, you could calculate the change in stored volume if you know the tank's dimensions or have a reasonably accurate level measurement.
Try to account for water that was intentionally diverted before the tank. A first-flush diverter, for example, sends the first part of a rainfall event away from storage to reduce debris and contaminants entering the tank.
Whether you include that diverted water depends on what you are trying to measure.
If you want the performance of the roof itself, measure runoff before intentional diversion where practical.
If you want the effective coefficient for your complete collection system, include those losses.
Step 4: Divide Runoff by Rainfall Volume
Suppose the theoretical rainfall volume was 623 gallons, but you measured 530 gallons of runoff.
The calculation is:
530 ÷ 623 = 0.85
Your measured runoff coefficient would therefore be about:
0.85
You could also express this as:
85% runoff efficiency
Runoff Coefficient Formula
The general formula is:
C = V ÷ (P × A)
Where:
- C = runoff coefficient
- V = measured runoff volume
- P = rainfall depth
- A = catchment area
All units must be compatible.
If you calculate the theoretical rainfall volume separately first, the simpler form is:
C = actual runoff ÷ theoretical rainfall volume
The runoff coefficient normally falls between 0 and 1.
A value close to 1 means most of the rainfall became runoff.
A lower value means more water was lost before reaching the measurement point.
Example in Metric Units
Metric calculations are especially simple because:
1 millimeter of rain on 1 square meter = 1 liter of water
Suppose you have:
- 80 m² of roof
- 20 mm of rain
- 1,360 liters collected
The theoretical rainfall volume is:
80 × 20 = 1,600 liters
Now calculate the coefficient:
1,360 ÷ 1,600 = 0.85
The measured runoff coefficient is:
0.85
For future estimates from similar rainfall events, you could calculate:
Expected collection = roof area × rainfall × runoff coefficient
In this example:
80 × 20 × 0.85 = 1,360 liters
Why the Runoff Coefficient Is Less Than 1
Not every drop that lands on a roof reaches your tank.
Losses can come from several places.
Roof Wetting
A dry roof absorbs or holds a small amount of the first rainfall.
Water can remain as a thin film on roofing material, in seams, or around debris.
Evaporation
Some water can evaporate during light or intermittent rain, especially on warm roofing surfaces.
Splashing and Wind
For water volume from one inch of rain per square foot, first coordinate the influence of roof material on usable rainwater.
Wind can move rain away from the roof or gutters. Water may also splash over gutter edges during heavy rainfall.
Gutter and Downspout Losses
Poor gutter slope, leaks, overflowing gutters, blocked downspouts, and undersized drainage can reduce the amount reaching storage.
First-Flush Diversion
A first-flush system intentionally removes some early runoff.
This improves the quality of water entering storage but reduces the amount stored.
Filters and Screens
Leaf screens and other prefiltration components can also reject a small amount of water, especially if dirty or overloaded.
Should I Use a Standard Coefficient or Measure My Own?
For an early tank-sizing estimate, people often use an assumed runoff coefficient appropriate for the catchment surface.
For a more accurate estimate of your own system, measuring actual performance is better.
Roof material, slope, gutter design, rainfall intensity, debris, maintenance, and collection-system layout can all change the result.
Published runoff coefficients also serve different purposes. A coefficient used by an engineer for stormwater drainage is not always the same thing as the collection-efficiency factor you should use when estimating water entering a rainwater tank.
For rainwater harvesting, make sure the number represents the losses you actually want to include.
Measure More Than One Rainfall Event
One storm does not necessarily give you a reliable long-term coefficient.
A short, light rain may have a lower effective coefficient because a larger share of the rain is needed just to wet the roof.
A long, heavy storm may produce a higher coefficient once the roof and gutters are fully wet.
For a better estimate, measure several rainfall events under different conditions.
You can then calculate:
Overall coefficient = total runoff from all measured storms ÷ total rainfall volume from those storms
This is usually better than simply averaging the coefficients from individual storms because larger rainfall events should contribute more to the final result.
Make Sure the Tank Did Not Overflow
Tank overflow can make your calculation misleading.
Suppose 600 gallons reached your tank, but the tank had room for only 400 gallons. Measuring the tank level afterward would show only 400 gallons of additional storage.
Dividing 400 gallons by the rainfall volume would make the runoff coefficient appear much lower than it really was.
For testing, use rainfall events during which:
- The tank has enough spare capacity
- Overflow does not occur
- No water is being pumped out during the measurement period
- You can measure the starting and ending volume with reasonable accuracy
If water is used during the storm, add that amount back into your runoff measurement if you can measure it.
Account for First-Flush Water Consistently
Decide what your coefficient is supposed to represent before doing the test.
For example, imagine:
- 600 gallons fall on the roof
- 550 gallons leave the roof
- 20 gallons are sent through a first-flush diverter
- 530 gallons reach the tank
The roof-only runoff coefficient would be:
550 ÷ 600 = 0.917
The complete collection-system coefficient would be:
530 ÷ 600 = 0.883
Neither figure is automatically wrong. They describe different parts of the system.
For tank sizing and water-supply planning, the second figure is usually more useful because it represents the water you can actually store.
Runoff Coefficient vs. Collection Efficiency
These terms are sometimes used interchangeably, but they do not always mean exactly the same thing.
A runoff coefficient generally describes the fraction of rainfall that becomes runoff from a surface.
A collection efficiency may include additional losses from the entire rainwater system, such as:
- First-flush diversion
- Screens
- Filters
- Gutter overflow
- Pipe losses
- Tank overflow
When estimating tank inflow, a whole-system efficiency factor may give you a more realistic result than a roof-only runoff coefficient.
Always check how a coefficient is defined before using it in a calculation.
Using the Coefficient to Estimate Rainwater Collection
Once you have a reasonable coefficient, you can estimate future collection.
In U.S. units:
Gallons collected = roof area in sq ft × rainfall in inches × 0.623 × runoff coefficient
For example:
- Roof area: 1,500 sq ft
- Rainfall: 2 inches
- Runoff coefficient: 0.85
Calculation:
1,500 × 2 × 0.623 × 0.85 = about 1,589 gallons
That is an estimate of water reaching storage, assuming the coefficient includes the relevant collection losses and the tank has enough capacity.
In metric units:
Liters collected = roof area in m² × rainfall in mm × runoff coefficient
For an 80 m² roof receiving 25 mm of rain with a coefficient of 0.85:
80 × 25 × 0.85 = 1,700 liters
Do Not Treat the Coefficient as a Fixed Guarantee
A runoff coefficient is a planning value, not a promise of how much water every storm will produce.
Actual collection changes with:
- Rainfall intensity
- Storm duration
- Time since the previous rain
- Roof condition
- Leaves and debris
- Gutter condition
- Wind
- First-flush settings
- Filter condition
- Tank capacity
For water-supply planning, leave room for these real-world differences rather than assuming every rainfall event will match your calculated coefficient exactly.
Frequently Asked Questions
What does a runoff coefficient of 0.9 mean?
It means about 90% of the rainfall volume used in the calculation becomes measured runoff. The remaining 10% is lost through wetting, evaporation, splashing, diversion, or other losses included in your measurement.
Can a runoff coefficient be greater than 1?
Under normal conditions, it should not be. A result above 1 usually points to an error in rainfall measurement, catchment area, tank-volume measurement, unit conversion, or the identification of the area draining to the tank.
Is the runoff coefficient the same for every roof?
No. Roofing material, slope, roof condition, rainfall pattern, gutter design, and other factors can affect runoff. The effective coefficient for an entire rainwater system can also change because of first-flush and filtration losses.
Should I include first-flush water in my calculation?
For estimating how much water reaches your tank, yes. Treat first-flush diversion as a system loss. If you are studying only the roof's runoff behavior, you may calculate the roof coefficient separately.
How many rainstorms should I measure?
Several events are more useful than one. Include different rainfall amounts if possible. Calculate the coefficient from total runoff divided by total theoretical rainfall volume across all measured events.
Why was my coefficient lower during a small rainstorm?
A small rainfall event may spend a larger share of its water wetting the roof, gutters, and pipes. Light rain may also experience proportionally greater evaporation and other losses.
Can I use the runoff coefficient to size my rainwater tank?
Yes, it can be part of the calculation. Tank sizing also depends on roof area, local rainfall patterns, water demand, dry periods, available space, overflow management, and how much reserve you want. A runoff coefficient alone cannot determine the correct tank size.
