Daily rainfall simulation

Rainwater tank calculator.

Compare tank sizes using historical NASA rainfall or your own daily data, roof area and intended water use.

Try a garden example first → Synthetic rainfall, ready-to-use inputs.

Illustration: a roof refills one rainwater tank, which supplies the garden or non-potable home use. — Rainwater collection and use diagram. When animated, this uses the calculator's first tank size, roof, collection efficiency, starting fill and daily amount. NASA history supplies a representative month for the selected location when available; a clearly labelled example is used otherwise. Garden skips wet days; Home uses water daily; Both splits the entered daily total equally. This picture is not a weather forecast.

Free to use No account No tracking cookies

Preliminary, non-potable planning. Method & limits →

Start with your question

Choosing a tank?Compare sizes using your roof, rainfall and water use. Already have a tank?Check stored water, forecast refill and gravity pressure. Understand your water use?Estimate roof yield and garden or toilet demand.

Compare tank sizes

Units

Default point: Oslo, Norway. Choose your location.

Choose on map

Click the map to choose a location.

Coordinates & rainfall history

Latitude Longitude Rainfall history

Rainfall source & data

NASA rainfall Rainfall CSV

NASA provides historical daily rainfall for your chosen point. Choose CSV to use your own records.

12 monthly averages

Historical monthly averages from NASA POWER, not a weather forecast.

Avg high Avg low Precipitation

Monthly values

Month	Precipitation	Avg high	Avg low	Years: rain / temperature

Roof area (m²)

Quick estimate

Use the roof area that drains to your tank.

Roof area help

Presets are rough rectangular footprints, not measured house averages. Use only the horizontal area draining to your tank and refine it before buying. If you know the dimensions, enter them here:

Length (m) Width (m)

Tank sizes & collection settings

Collection efficiency (%) Starting water level (%) Sizes to compare (litres)

Examples & input guide

Try an example

Compare two synthetic rainfall patterns with the same yearly total. These are demonstrations, not local rainfall records.

Or start with editable settings for a practical use:

See each case and its assumptions

See the worked example and data

Input guide

Historical NASA rainfall: Daily rainfall estimates from NASA POWER for the regional grid around your coordinates. These are historical data, not a forecast or measurements from your property. The planner uses complete calendar years. Local gauge or station records can be uploaded instead.
Roof area: The horizontal area of the roof draining into your tank, rather than its sloped surface area.
Collection efficiency: The share of roof rainfall that reaches the tank after losses from first flush, filters and splash. A common planning range is 75–90%.
Daily water use: Your average daily demand for non-potable uses, such as irrigation or toilet flushing where locally permitted. Select months if the use is seasonal. The garden helper divides your weekly watering by seven; the toilet helper multiplies people, flushes per day and water per flush.
Starting water level: How full the tank is on the first day of the simulation. Its influence usually decreases over a long history.
Tank sizes: Enter 1–12 sizes separated by commas. The planner highlights the smallest tested tank within two coverage points of the best result.

Read the calculation method

Modeled result

Your tank comparison

Share or export

Size worth comparing

—

Why this size?

—

What the numbers mean

Demand covered: Share of the modeled water volume supplied.
Overflow / year: Annualized water lost when incoming rain exceeds available tank space.
Days fully supplied: Share of days with water use that have no unmet demand.
Water supplied / year: Annualized average across the historical record.
Longest shortfall: Consecutive days with any unmet demand.

Check garden watering →

Low, normal & high water use

Same tank, three water-use cases
Water use	Demand covered	Longest shortfall	Size for this use

How to read this

Normal is your entered water use. Low uses 25% less and high uses 25% more, during the same active months. Rainfall, roof area and other settings stay the same. Coverage and shortfalls compare the tank shown above; the last column shows the size highlighted for each case.

These are illustrative cases, not a forecast. Each case applies the same near-plateau rule; a highlighted size can still have shortfalls. If you used a roof preset, try another roof area too before choosing a tank.

Coverage by tank size

The curve shows when additional storage adds little modeled benefit.

Rainfall & assumptions

A grid cell is not a rain gauge at your roof. Use local measured data when the purchase decision depends on site-specific accuracy.

All tank sizes & detailed results

Side-by-side tank scenarios
Tank	Demand covered	Water-use days fully supplied	Water supplied / year	Overflow / year	Longest shortfall

Planning estimate. Check local rainfall and installation requirements before buying.

Limits & buying checklist

Before choosing a tank

Check water useReview the low and high cases above.

Check rainfallCompare with local measured data.

Check installationBuying checklist →

Find tanks around this size

Check dimensions, delivery and installation with the supplier.

Delivery country

Choose a delivery country to see links.

About these supplier links

Independent links, not paid rankings. Check delivery, dimensions and installation requirements with the supplier. How these links are selected

Was this useful?

Why this is more than a formula

Tank value depends on timing, not only total rain.

Collect

One millimetre of rain over one square metre equals one litre before losses. The model applies your efficiency to every daily rainfall value.

Store and spill

Each day’s inflow enters the tank. Water above the tested capacity becomes overflow, so larger tanks can retain more between wet and dry periods.

Draw and compare

Your daily water use is deducted from each capacity during the months you choose. The output compares coverage, water-use days fully supplied, overflow, and the longest modeled shortfall.

Inspect the complete method, assumptions, source quality, and recommendation rule →

Planning library

Make the assumptions before comparing tanks.

Practical help with roof area, water use, rainfall and installation.

Start here

What size rainwater tank do I need?

Turn a water-use goal into several defensible scenarios, then interpret coverage, overflow, and dry-period shortfalls.

Source quality

Choose and check daily rainfall data

Understand the tradeoff between global grid data and a local gauge, including gaps, units, date ranges, and false precision.

Input quality

How much rainwater can my roof collect?

Estimate roof collection and water use, then check how long stored rainwater lasts or what the next rain could refill.

Before purchase

Ten checks before buying a rainwater tank

Turn the modeled capacity into a brief covering access, foundations, overflow, plumbing, water quality, maintenance, and local rules.

Common questions

What the result can—and cannot—tell you

How much rainwater can I collect from my roof?

Potential collection in litres is roof footprint in m² × rainfall in mm × collection efficiency. For example, a 100 m² roof with 600 mm of rain and an illustrative 80% efficiency gives 48,000 L before tank overflow. The water you can actually use depends on rainfall timing, storage and demand. See the roof-yield calculation and unit conversions.

Is a 5,000 litre rainwater tank big enough?

It depends on the roof, rainfall, dry spells and water use. In our synthetic garden example, a 3,000 L tank covers 98.1% of demand and 5,000 L covers 100%; those figures do not predict performance at your property. Compare the worked scenarios, then run your own inputs.

Does the highlighted capacity mean that is the tank I should buy?

No. It is the smallest capacity you entered that falls within two percentage points of the best modeled demand coverage. Space, plumbing, water quality, regulations, structural loads, drought resilience, price, and local professional advice can change the decision. Follow the rainwater tank sizing guide before choosing a product.

Why use daily rainfall instead of annual rainfall?

Two places can receive the same yearly total with very different wet and dry sequences. Storage helps only when water captured earlier remains available when demand occurs later. In our reproducible 600 mm example, a 500 L tank supplies 100% of demand with frequent rain, but only 16.4% with monthly storms. The roof and demand are identical.

Can this plan drinking-water storage?

No. The current scope is non-potable preliminary planning. Roof-water quality, treatment, backflow protection, materials, inspection, and legal requirements need local qualified guidance.

Is NASA POWER rainfall exact for my property?

No. It is modeled, gridded climate data. It is useful for a consistent global screen, but station or gauge records closer to the property may be better. Uploading a daily local CSV lets you compare.

Can I use this for a rain barrel, water butt, or cistern?

Yes, if it collects roof rainwater for a simplified non-potable demand. Enter the actual candidate volumes in litres or US gallons. Small barrels may overflow frequently, while large cisterns need additional site, structural, plumbing, water-quality, and local-rule checks.