Every rainfall counts. Estimate what your roof can collect.

From roof to tank: calculate the recoverable volume and what your facility can actually store.

Your scenario, your settings

Rainfall, roof area, collected volume.

The basic rule is simple: 1 mm of rain on 1 m² yields 1 litre before losses. Enter only the roof area connected to your collection system.

01Your collection area

Input unitsHow will you enter the roof area?

Length (m)Width (m)

Measure the horizontal projection to the ground, not the inclined surface of the panels. Do not climb on the roof to measure.

Share of this connected area (%)

Keep 100% if the surface is already limited to the descents used.

02Rain and loss

Rain height (mm)Collection efficiency (%)

25 mm is a calculation scenario, not an average rainfall or a forecast. The 75% collection efficiency is a PrépaCivile planning assumption; adjust it to the actual losses in your system.

Additional first-flush diversion before storage (L)

Optional, for a known diversion of the first waters. Leave 0 if this loss is already included in the yield, not to count it twice. Diversion does not prove potability.

03Your storage capacity

Total tank capacity (L)Water already present (L)

For several communicating tanks, add only their actually usable capacities. The model does not simulate a consumption during the shower.

Local calculation, without account, without backup or transmission of your entries.

Enable JavaScript for the interactive calculation. Volume before storage = connected roof area (m²) × rainfall (mm) × collection efficiency, minus any additional first-flush diversion.

Understanding · Checking · Acting

How to use your result.

Find the method, useful actions and limitations of the calculation in the same place, on each PrépaCivile tool.

Calculation method

What if the shower was different?

These scenarios use your surface, yield, diversion and available space. They do not represent a showering frequency or local forecast.

Comparison of four heights of rain
Rain	Recoverable	Stockable extra

Understand the calculation.

Connected area = horizontal area × connected part.
Theoretical volume = surface connected in m2 × rain in mm.
Recoverable volume = theoretical volume × yield - additional diversion, without descending below zero.
Additional storage volume = minimum between recoverable volume and available space.
Surplus = recoverable volume - additional storage volume.

Example: 80 m2 × 25 mm × 75% = 1,500 L before the tank. With an empty barrel of 220 L, only 220 L are stored: 1,280 L exceed the available capacity.

Practical next steps

Some benchmarks to prepare your installation

Use a measured rain or explicit scenario. An annual average cannot estimate availability at a given date.
Keep the tank closed, with appropriate protection against debris and mosquitoes.
Provide stable support, overflow and gel protection depending on the installation.
Keep drinking and non-drinking water systems separate. For indoor connections, have the requirements of a qualified person checked.
The slope does not replace the horizontal projection in the volume calculation, but the material, roof condition and maintenance can influence water loss and quality.

Limitations of the result

This result concerns a shower and installation described by your parameters. It does not prove the potability of water, nor an annual autonomy. The yield and storage volume remain assumptions to be checked.

Sources and references

CMHC — Collecting and Using Rainwater at Home (2013, English PDF) : volumetric principle, area of collection and limits of storage. Technical reference document, not confirmation of current local rules.
Health Canada — Mosquitoes Prevention of stagnant water and protection of barrels.

Sources consulted November 1, 2026. Estimation for a liquid rain shower, without snowmelt, hydraulic simulation, performance guarantee, quality diagnosis or regulatory validation.

Frequently Asked Questions

Why does the calculation use the horizontal surface?

The rain is expressed in height on a horizontal surface. Use the length and width seen from the top, only for the part connected to the descents used.

Is collection efficiency always 75%?

No. It's an adjustable starting hypothesis. Losses depend on roof, gutters, filters and bypasses, among other things. It doesn't deduce guaranteed performance.

Can I drink that water?

The calculator does not make it possible to conclude that it is safe to drink. It estimates a volume. Do not assimilate to a supply of drinking water; uses and treatments require adequate verification.

Can I provide an annual autonomy with this result?

No. A single shower does not describe dry periods, consumption or storage over the seasons. An annual dimensioning requires time simulation and local data.

Is my information recorded?

No. The calculations stay in your browser, without counting or sending fields. Reloading restores the default scenario.

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