Roof Rainwater Flow Rate Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/4/2026
The collected water flow rate is found by multiplying the roof surface area by the rainfall intensity (1 mm of rain over 1 m² equals exactly 1 liter). For a 100 m² roof under a 60 mm/h storm, the flow rate reaches 100 liters per minute.
Explanation
Correctly sizing a gutter or downspout always starts with the same question: how much water volume does it need to carry away per unit of time? The conversion between rainfall intensity and flow rate rests on an exact equivalence, not an approximation: one millimeter of rainfall over one square meter represents exactly one liter of water, regardless of the surface area considered. So multiplying the roof surface area (in m²) by the rainfall intensity (in mm/h) gives a flow rate in liters per hour, which is then divided by 60 to express it in liters per minute, the more common unit for sizing drainage. This calculator deliberately stops at this flow rate: choosing the gutter diameter or the number of downspouts needed then depends on normative tables (in France, standard NF EN 12056-3) and the precise specifications of the installed product, which a simple flow-rate calculation cannot replace — consult those references or a professional for the final sizing. To find the real surface area of a sloped roof section from its footprint, see our sloped roof surface area calculator; for another volume-planning calculation on the same building project, see our concrete volume calculator.
Example: a 100 m² roof under a 60 mm/h storm
Inputs
Roof surface area: 100 m². Rainfall intensity: 60 mm/h.
Calculation
Hourly flow = 60 mm/h × 100 m² = 6,000 L/h. Flow per minute = 6,000 ÷ 60 = 100 L/min.
Result
This roof needs to drain about 100 liters per minute during a storm this intense.
Frequently asked questions
What rainfall intensity should I use for safe sizing?
Professionals generally size for an intense but realistic rainfall event for the region (often around 60 mm/h in France for a heavy storm, sometimes more depending on local weather data), rather than for an average daily rainfall. Check your region's rainfall data or consult a building professional for a reliable sizing value.
Does this calculator directly give me the gutter diameter to choose?
No, deliberately so: choosing a diameter depends on normative tables (NF EN 12056-3 in France) that combine the flow rate to be drained, the gutter's installed slope, and the number of downspouts planned. This calculator provides the flow rate to be drained, the input those tables need, but doesn't replace them.
Why does 1 mm of rain over 1 m² equal exactly 1 liter?
Because one millimeter equals 0.001 meter: a 1 mm deep sheet of water over a 1 m² surface represents a volume of 0.001 m × 1 m² = 0.001 m³, exactly 1 liter (1 m³ = 1,000 L). This equivalence is an exact unit conversion, valid regardless of the surface area considered.