Rainwater Tank Sizing & Bridge the Dry Spell
Sizes storage
Enter your daily water demand, the longest dry spell and a safety margin to get the base volumeand the recommended tank size in litres and m³ — enough to carry you to the next rain.
Rainwater tank sizing
Next: install a tank of at least 36,000 L (36 m³) so 2,000 L/day keeps flowing through a 15-day dry spell.
Base reserve = daily demand × dry-spell length; the safety margin covers demand spikes, evaporation and an imperfectly full start.
Rainwater tank sizing — key facts
- Base volume
- daily demand × dry-spell days
- Tank size
- base × (1 + safety margin)
- Safety margin
- ≈ 10–25%
- 1 m³
- = 1,000 litres
- Undersized
- Runs dry mid-season
- Oversized
- Costly; water can stagnate
- Pair with
- Catchment & harvesting sizing
- Privacy
- Runs in your browser; nothing uploaded
Size storage to the longest gap, not the average
A storage tank earns its keep during the dry spell — the stretch when no rain falls and no refill arrives, yet your crop, livestock or household still need water every day. Size the tank to that gap: multiply your daily demand by the number of dry-spell days, then add a margin for spells that run longer or use that runs higher than planned. Sized to the average, a tank empties at the worst moment; sized to the longest credible gap, it carries you through.
This tool computes the base volume, the recommended tank size in litres and in m³, and the dry-spell days it covers. Get the buffer right and you decouple your water supply from the timing of the next rain. Pair it with the Rainwater Harvesting, Water Tank Capacity, Farm Pond and Tank Command Area tools to design the catchment and distribution around the storage.
Bridge the dry spell
Storage to last from one rain to the next.
Plan for the worst gap
Size to the longest dry spell, not the mean.
Build in a margin
Cover longer spells and demand spikes.
Right-size, not oversize
Enough buffer without wasted cost or stale water.
Frequently Asked Questions
How do I size a rainwater storage tank?+
A tank must hold enough to bridge the longest dry spell between rains or refills. The base size is your daily water demand multiplied by the number of dry-spell days; you then add a safety margin for unusually long gaps or higher-than-expected use. This tool does both and gives the recommended capacity in litres and m³.
What is the sizing formula?+
Base volume = daily demand × dry-spell days, and tank size = base volume × (1 + safety margin). For example, 800 L/day across a 30-day dry spell is a 24,000 L base; with a 20% margin the recommended tank is about 28,800 L (≈28.8 m³).
How do I work out my daily demand?+
Add up everything the tank must supply each day — irrigation top-ups, livestock watering, washing and household use. For irrigation, daily demand often comes from the crop's ETc over the area divided by your system efficiency. Enter the realistic peak daily figure so the tank covers your busiest period, not the average.
What dry-spell length should I use?+
Use the longest stretch you realistically need to ride out between refills — the gap between reliable rains in your area, or the interval between deliveries or pump runs. Local rainfall records and your own experience are the best guide. Sizing to the longest credible dry spell is what keeps you from running empty mid-season.
Why add a safety margin?+
Real conditions vary: a dry spell can run longer than usual, demand can spike on hot days, and tanks are rarely drawn down to the very last drop cleanly. A margin of 10–25% absorbs that variability so a slightly worse-than-planned season does not leave you short. Higher-risk situations justify a larger margin.
What happens if I undersize the tank?+
An undersized tank runs dry before the next rain or refill, and that often coincides with the hottest, driest part of the season when the crop or livestock need water most. The cost of being short — lost yield or stressed animals — usually dwarfs the modest extra cost of a tank sized with a sensible margin.
Can a tank be too big?+
A larger tank costs more up front and takes more space, and water that sits a long time can stagnate or grow algae if it is not turned over and kept clean. The aim is enough storage to bridge the dry spell with a reasonable margin — not the largest tank you can buy. This tool targets that practical size.
Should I match the tank to my catchment?+
Tank size and catchment work together: the tank must hold enough to last the dry spell, while the catchment (roof or area times rainfall and runoff) must refill it during the wet periods. Size the tank here for demand, then check that your harvesting area can actually fill it with the Rainwater Harvesting tool.
Does this work for household, livestock or irrigation use?+
Yes — the method is the same for any use. You only need the daily demand and the dry-spell length. Whether the tank serves a house, a herd, a nursery or a field, multiplying realistic daily use by the days you must cover (plus a margin) gives the storage you need.
How does this fit my wider water plan?+
Tank sizing is one piece of farm water security alongside harvesting catchment, ponds and command-area planning. Size the buffer tank here for your dry-spell demand, then use the Rainwater Harvesting, Water Tank Capacity, Farm Pond and Tank Command Area tools to design the catchment and distribution around it.