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Rainwater harvesting calculator

Estimate how much rainwater you can collect from a roof area and rainfall.

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Enter Roof area (m²), Rainfall (mm) and the Rainwater harvesting calculator works out Water collected (litres) straight away. For instance, with Roof area (m²) = 100 and Rainfall (mm) = 50 it returns Water collected (litres) = 4,500.

How to use it

  1. Enter your values: Roof area (m²), Rainfall (mm).
  2. Read the result instantly: Water collected (litres).

Frequently asked questions

How does the Rainwater harvesting calculator work?

It takes Roof area (m²) and Rainfall (mm) and derives Water collected (litres) from them. The calculation is live as you type, so the result updates on every change.

Which values does the calculator ask for?

2 values: Roof area (m²) and Rainfall (mm). Nothing else is required — no account, no file upload.

What does a typical calculation look like?

With Roof area (m²) = 100 and Rainfall (mm) = 50, the calculator returns Water collected (litres) = 4,500. Those figures come from running this exact tool, so you can reproduce them by entering the same values.

How much does the result change with different inputs?

It moves a lot. Using Roof area (m²) = 110 and Rainfall (mm) = 55 instead, Water collected (litres) goes from 4,500 to 5,445 — which is why it is worth testing a few scenarios rather than trusting a single figure.

What does it give for smaller values?

Scaled down to Roof area (m²) = 90 and Rainfall (mm) = 45, Water collected (litres) comes out at 3,645. The relationship is worth checking at both ends before you rely on a single result.

When would I actually use this?

Buying the right amount at the merchant: how many tiles, how much paint, how many bags of concrete, and what it all comes to.

What is the most common mistake?

Ordering the exact calculated quantity. Every material loses some of itself between the pallet and the finished job — offcuts, breakage, a bag that sets, a coat that soaks in further than the datasheet says — so add the allowance yours asks for, and order it in one delivery: a second batch rarely matches the first.

How accurate is it, and what are the limits?

Estimate only — confirm with a professional.

What is the difference between the Rainwater harvesting calculator and the Roofing shingles calculator?

This one returns Water collected (litres); the Roofing shingles calculator returns Shingle bundles. That is the whole difference — open the one whose figure you need.

Is there a tool for the next step?

Deck stain calculator is the closest one after this: How much stain a deck needs: the surface area of the boards, the railing and the stairs, divided by the coverage of your stain — which drops for solid stains and for rough or weathered wood — times the number of coats. Enter dimensions and it returns the gallons and, optionally, the cost.

Further reading

All guides →
How-toHow to Calculate Pool Volume: A Formula per Shape, and the Depth TrapLength × width × average depth gives 21,065 gallons for a 32 by 16 ft pool. But average depth only means (shallow + deep) ÷ 2 when the floor slopes evenly, and most pools do not. Here is every shape formula and the correction.How-toHow Many Wall Studs Do I Need? Framing by Spacing and LengthCount the studs a wall needs from its length and stud spacing, then add for corners, openings and end studs so nothing is missing on framing day.How-toHow Many Concrete Blocks Do I Need? Blocks, Mortar and WasteWork out how many concrete blocks a wall needs from its area, add mortar and a waste allowance, and avoid a second trip to the yard.How-toHow Much Roofing Material Do I Need? Area and SquaresTurn your building footprint into true roof area using the roof pitch, then convert to roofing squares or square meters plus a waste allowance.How-toHow Many Deck Boards Do I Need? Coverage and GapsWork out deck boards from the deck area, each board's covered width including the gap, and a waste allowance for cuts and the picture-frame border.How-toHow Much Fencing Do I Need? Perimeter, Posts, PanelsMeasure your run, then work out posts, panels, and rails from the perimeter and a standard post spacing so nothing is short on install day.