Rainfall to Volume and Flow Rate Converter

Example preset

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Inputs

area

Rainfall (total rainfall)

*Here, enter the "total (total rainfall) within the period".

Trivia: 1 mm of rain is approximately 1 L per 1 m² of water.

Also calculate runoff volume (advanced settings)

duration

If not entered, the average flow rate (m³/s) will not be calculated (required for back calculation of average flow rate → rainfall amount).

Loss (permeation, storage, etc.)

Runoff coefficient C (0~1)

Results are automatically updated on input changes (can be restored with share URL).

Results

Area (m² conversion)	—

—

Effective rainfall (after deducting losses)	—
Runoff volume (approximate)	—
Average flow rate (approximate) i *The flow rate is averaged over the entire period of rain. This is different from the instantaneous maximum flow rate (peak).	—

Calculation formula (overview)

Show calculation formula

volume: V = (rain_mm/1000) × area_m²

effective rainfall: effective = max(rain_mm − loss_mm, 0)

Runoff volume: V_runoff = (effective/1000) × area_m² × C

average flow rate: Q_avg = V_runoff / (duration_h × 3600)

Guidelines and examples

These are general examples only. Values vary with land cover, slope, prior rainfall, and drainage conditions.

Example use cases

Estimate how much rooftop rainwater can be stored in a tank.
Estimate total rainfall volume over a schoolyard or parking area.
Estimate basin runoff during heavy rain as a first-pass check.

Estimated runoff coefficient C (reference)

City (reference)	0.8 (example when there are many impermeable surfaces)
Suburbs (reference)	0.6 (example of mixed land use)
Farmland (reference)	0.5 (varies depending on crop and soil conditions)
Forest (reference)	0.3 (example when infiltration is large)

*This table does not indicate the "correct answer". If necessary, please make adjustments based on literature, local government materials, and local conditions.

Frequently asked questions

How much water is "50mm of rain"?

If 50mm of rain falls on an area of 1km², the volume will be 50,000m³(= 50 million L).

What is runoff coefficient C?

This is the ratio (0 to 1) of how much of the effective rainfall comes out as runoff. It varies greatly depending on land use and ground conditions.

Is the "flow rate" shown here the maximum flow rate?

No. The flow rate shown here is the average flow rate over the selected period. Peak flow requires a separate hydrologic model.

How should I use the runoff coefficient?

Choose a coefficient that matches the land cover and drainage assumptions, then test a range such as paved, mixed, and forested conditions instead of treating one value as exact.

Why can this result differ from a flood model?

This page converts rainfall depth, area, loss, coefficient, and duration into total volume and period-average flow. It does not estimate peak discharge, routing delay, or stormwater network capacity.

How to use the rainfall volume and runoff calculator effectively

What this calculator does

This page converts rainfall depth and area into total water volume. When you enter duration, losses, and runoff coefficient C, it also estimates effective rainfall, runoff volume, and period-average flow rate.

Input meaning and unit policy

Rainfall is entered as total depth for the selected event, not as intensity. Area units are converted to square metres, duration is converted to seconds, and losses are subtracted before applying the runoff coefficient.

Runoff coefficient guidance

Treat C as a scenario assumption. Paved surfaces usually need a higher value than forest or farmland, while slope, soil moisture, drainage, and rainfall intensity can shift the result. Compare several plausible values instead of relying on one exact coefficient.

Common mistakes to avoid

Do not read the average flow as peak discharge. The result also does not include routing delay, storm sewer capacity, infiltration modelling, or local design standards, so engineering or safety decisions need a separate hydrologic review.

Interpretation guidance

Use total volume for storage or capture checks, effective rainfall for loss assumptions, runoff volume for first-pass basin estimates, and average flow for rough period-level comparisons. Document the event duration and coefficient when sharing results.

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