RTC

Ecology & Environment

Rainwater Tank Capacity Calculator

Check event capture, first-flush loss, usable tank storage, overflow, dry-period demand, capacity shortfall, supported days, and required nominal tank size.

Effective roof area (m2)Design rainfall event (mm)Runoff efficiency (%)First-flush diversion (L)Installed tank capacity (L)Average daily demand (L)Design dry period (days)Operational reserve (%)

Gross event capture-

Capture after first flush-

Tank capacity after reserve-

Water stored from event-

Event capture above usable capacity-

Demand across design dry period-

Dry-period demand minus usable capacity-

Days supported at entered demand-

Nominal tank needed including reserve-

Decision view

Reserve-adjusted rainwater tank section

Reserve-adjusted rainwater tank sectionNet event inflow, stored water, reserve, overflow, and dry-period demand are placed on one capacity cross-section.

Exact scenario comparisonInstalled tank capacity (L) changes while all other entered assumptions remain constant.

Installed tank capacity (L)	Gross event capture	Capture after first flush	Tank capacity after reserve	Water stored from event	Event capture above usable capacity	Demand across design dry period	Dry-period demand minus usable capacity	Days supported at entered demand	Nominal tank needed including reserve

How to use Rainwater Tank Capacity Calculator

Enter roof area, design rainfall, efficiency, and first flush.
Enter nominal tank size and reserve percentage.
Enter daily demand and design dry days.
Read the live tank section, overflow, supported days, and required size.

Calculator guide

Understanding Rainwater Tank Capacity Calculator

A rainwater tank must be checked against two different constraints: how much a design event can deliver and how much water a dry period requires. First flush and operating reserve reduce those quantities at different points, so combining them into one percentage can hide overflow or shortfall.

Two constraints Event inflow and dry demand are separate checks.

Reserve reduces storage It does not reduce rainfall.

Overflow is explicit Excess event water is not stored.

Required size is nominal Reserve is added back in the denominator.

Calculation method

How the calculation works

Translate one design rainfall event into gross and net capture, enforce tank reserve and capacity, then compare usable storage with a stated dry-period demand. First flush is deducted before storage, while overflow and reserve-adjusted dry-period capacity remain distinct. Calculate event litres from roof rainfall, subtract first flush, reduce nominal tank capacity by reserve, then compare both event inflow and dry-period demand with usable storage.

Detailed calculation process

Reconcile design-event inflow with usable storage and dry-period demand

The default design uses a 160 m2 roof, 35 mm event, 80% runoff efficiency, 120 L first flush, a 5,000 L tank, and 18 dry days at 280 L/day.

General formula: G = A R etaN = max(G-F,0)U = T(1-r)S = min(N,U)O = max(N-U,0)D_d = qdQ = max(D_d-U,0)T_req = D_d/(1-r) The event first creates gross capture, first flush reduces inflow, and reserve reduces usable tank space. Stored water is the smaller of net inflow and usable capacity, while dry-period sufficiency compares demand with usable capacity independently.

What each symbol means

A, R, eta Roof area (m2), event rainfall (mm), and efficiency.

F First-flush diversion (L).

T, r, U Nominal tank size, reserve share, and usable capacity (L).

G, N, S Gross capture, net event capture, and stored event water (L).

O Event overflow above usable storage (L).

q, d, D_d Daily demand, dry days, and dry-period demand.

Q, T_req Capacity shortfall and required nominal tank size (L).

Worked substitution with the default inputs

1. Calculate gross event capture eta = 80/100 = 0.80G = 160(35)(0.80) = 4,480 L The mm-m2 litre identity converts the design storm directly.

2. Subtract first flush N = max(4,480-120,0) = 4,360 L First flush reduces incoming event water, not the physical tank volume.

3. Reserve operating volume r = 10/100 = 0.10U = 5,000(1-0.10) = 4,500 L The entered reserve leaves 90% of nominal storage available.

4. Check event storage S = min(4,360,4,500) = 4,360 LO = max(4,360-4,500,0) = 0 L The design event fits inside usable capacity after first flush.

5. Check the dry period D_d = 280(18) = 5,040 LQ = 5,040-4,500 = 540 LT_req = 5,040/0.90 = 5,600 L The installed tank supports 4,500/280 = 16.071 days and is 600 L nominally smaller than the dry-period target.

The default event fits without overflow, but usable storage is 540 L short of 18-day demand; a 5,600 L nominal tank would meet that reserve-adjusted target.

Engineering storage view

See event inflow fill the reserve-adjusted tank

A tank cross-section separates stored water, available headroom, and reserve, with external markers for event inflow and dry-period demand.

Net event inflow Capture after first flush.

Stored volume Limited by usable capacity.

Reserve zone Unavailable entered percentage.

Demand marker Required water for the dry period.

Worked situations

Practical examples

The design event supplies 4,360 L after first flush.
A 5,000 L tank provides 4,500 L usable storage.
Eighteen dry days require 5,040 L, leaving a 540 L usable shortfall.

Better inputs

Useful tips

Keep first flush separate from the operating reserve.
Check event overflow and dry-period shortfall independently.
Round required capacity upward to an available compliant tank size.

Before relying on the result

Limitations and common mistakes

This is a single-event capacity check rather than a continuous rainfall-demand simulation.
Tank geometry, pump dead volume, fire reserve, losses, and connected demand may change usable capacity.
Local plumbing, structural, water-quality, and overflow requirements still apply.

Reference

Key terms

First flush: Entered initial runoff diverted before tank inflow.
Operating reserve: Entered share of nominal volume held unavailable.
Usable capacity: Nominal tank volume after reserve.
Dry-period demand: Daily demand multiplied by design dry days.

Important note

Calculated from the entered environmental values using the displayed model. Measurement quality, local conditions, system boundaries, and source data affect interpretation.

Frequently asked questions

Why is usable capacity only 4,500 L?

The entered 10% reserve is removed from the 5,000 L nominal tank.

Why is there no overflow?

Net event capture of 4,360 L is below usable storage.

Why is a larger tank still required?

The 18-day demand exceeds usable storage even though one event fits.

Does supported days include new rainfall?

No. It divides usable capacity by entered daily demand.