WaterCalcLabRainwater Tank Size Calculator
Size usable storage for a chosen rainless interval and compare annual supply with constant annual demand. Rainfall here must be annual.
Your result
Bars compare only results in the same unit group. They are normalized, not a time-series forecast.
Planning result based on your inputs.
How this tool works
Size usable storage for a chosen rainless interval and compare annual supply with constant annual demand. Rainfall here must be annual.
Vtank = demand/day × dry days ÷ usable fractionExample calculation
The prefilled form is a worked example. Its base-unit inputs and expected outputs are shown below; use the reset button to restore it.
| Input | Example |
|---|---|
| Daily demand | 100 |
| Days | 21 |
| Usable storage fraction | 0.9 |
| Area | 100 |
| Rainfall for the selected period | 600 |
| Roof runoff coefficient | 0.85 |
| System efficiency (separate losses) | 0.9 |
Planning notes for this calculator
Estimate nominal storage for a stated rainless interval. This is useful for comparing a dry-spell target with annual collection potential, before a detailed rainfall-and-demand simulation.
Choose meaningful inputs
- Daily demand
- Use water needed per day for the intended uses, preferably from a meter or watering log.
- Days
- Choose the dry interval to test, rather than using all days in a year.
- Usable storage fraction
- Allow for water below the outlet and space kept free. This is usable volume, not a collection efficiency.
- Area
- Measure the horizontal area that actually drains to the tank. Exclude disconnected roof sections.
- Rainfall for the selected period
- This tool requires an annual rainfall total for its separate annual balance.
- Roof runoff coefficient
- This fraction describes roof runoff before downstream collection losses. Use a documented assumption or a measured value.
- System efficiency (separate losses)
- This fraction applies to separate downstream losses. Do not count the same filter or first-flush loss in both coefficients.
Read the result correctly
Required capacity covers the dry-period demand after allowing for unusable storage. The annual balance separately compares collection with daily demand multiplied by 365. A positive balance does not prove the tank stays full through summer; the two calculations describe different time scales. A larger tank cannot remedy inadequate long-term supply.
Common mistakes to avoid
- Do not enter one storm total in the annual rainfall field.
- Do not assume the tank is full when a dry spell begins without checking the preceding rainfall.
A useful next step
Test several dry intervals and demand levels. Compare suitable tank sizes using local daily rainfall, starting fill and overflow.
Follow a worked scenario
A separate illustrative scenario; these values do not change when you edit the form above.
- 100 L/day × 21 days = 2,100 L usable storage needed.
- 2,100 ÷ 0.90 = about 2,333 L nominal capacity; annual collection of 45,900 L exceeds constant annual demand of 36,500 L by 9,400 L.
Frequently asked questions
Should I buy the exact calculated tank size?
Use it as a scenario target, then compare available products, site access, load-bearing requirements and rainfall timing. Nominal product capacity and usable capacity can differ.
Why show an annual balance as well?
It exposes whether your constant daily demand is plausible against annual collection. It cannot locate seasonal shortages; that needs a time-step water balance with overflow and withdrawals.
Continue your plan
Rainwater Storage Duration Calculator
Divide usable storage by a constant daily demand to estimate rainless supply duration.
Open calculator ↗Water storageWater Tank Volume Calculator
Calculate exact geometry from internal dimensions for a vertical cylinder, box or sphere.
Open calculator ↗Water storageWater Tank Fill Time Calculator
Estimate refill time from current contents and a constant inlet flow.
Open calculator ↗Related reading
Water storageUnderstanding Rainwater Overflow
A collection system needs somewhere for incoming water to go after the tank reaches its operating limit. Plan that route from the start rather than treating every overflow as a malfunction. Distinguish a full-tank overflow from water escaping at a blocked gutter or inlet; the latter can occur even while substantial storage space remains available.
Read guide ↗
Water storageHow to Calculate Tank Capacity
Geometry tells you the volume inside a shape. Sizing asks whether that volume matches supply and demand. Establish which question you need to solve. A correctly calculated cylinder volume can still be an unsuitable garden supply. Conversely, an appropriate capacity target needs product dimensions and operating levels to confirm how much water can actually be stored.
Read guide ↗
Water storageHow Long Does Stored Rainwater Last?
The question has two meanings: how long a volume can supply demand and how long water remains suitable for a particular use. Only the first can be answered with a simple volume calculation. Suitability depends on source water, storage conditions, treatment and exposure. A calculator showing a number of days is not a water-quality expiry date.
Read guide ↗
Water storageAbove-Ground vs Underground Water Tanks
Above-ground and buried storage solve different space problems. Sketch available surface area, entrances, utility routes, groundwater conditions and intended tank access. An underground option may preserve garden space, while a surface option may simplify inspection. Neither benefit establishes suitability on its own. Ground conditions and product-specific installation limits need professional assessment.
Read guide ↗
Rainwater harvestingRainwater Harvesting in Dry Climates
In a dry climate, a useful annual rainfall total can still arrive in a few events separated by long rainless periods. Describe the longest relevant gap and which uses must continue through it. Decide whether rainwater is a supplemental source or a supply expected to meet essential demand. These two objectives lead to very different acceptable risks and storage requirements.
Read guide ↗
Water storageHow to Choose a Rainwater Tank
Write a short specification describing the uses, required usable volume, outlet flow, installation space and maintenance access. A tank is one component of a system, so compare options against the same specification. A bigger container is not automatically better if it cannot be filled regularly, is difficult to service or needs a costly pump arrangement.
Read guide ↗
Rainwater harvestingHow Rainwater Harvesting Works
A rainwater system is easiest to understand as a route with checkpoints. Identify the connected roof, follow each gutter to its downpipe, locate the inlet and then trace the outlet to the watering point. A large roof disconnected from the tank contributes nothing. Draw this route before comparing storage products; it often exposes the missing connection that changes a project.
Read guide ↗Sources & method references
- Texas Water Development Board: Rainwater Harvesting Manual
Collection, storage and system-planning context. Geometry and unit conversions on this page are stated explicitly.
Method version 1.1 · Explanations and examples reviewed: 2026-10-11