Off-Grid Water Usage & Storage Calculator
Work out how much water your household uses each day, how much storage you need for a period without refills, and how long the water in your tank could last. This off-grid water calculator lets you compare everyday activities, a protected reserve, rainfall collection and scheduled refills in litres or US gallons.
Start with an editable household preset, then replace the example values with your own measurements. The results separate daily demand, usable storage and the timing of new water, so you can see where a plan falls short.
Jump to the water calculator
How to use this water calculator
- Set your household and planning period. Choose a preset as a starting point, then check the activity assumptions. Add outdoor uses only when they draw from the tank you are modelling.
- Describe your tank. Enter nominal capacity, current fill and a protected reserve. Advanced mode adds unusable capacity and fixed daily storage loss.
- Read the two storage answers separately. Required capacity assumes a full tank and no replenishment. Current autonomy uses the water available in your existing tank.
- Test arrivals in Advanced mode. Add rainfall or scheduled refills and inspect the daily table for unmet demand and overflow. A positive overall water balance can still hide days without enough water.
Off-Grid Water Usage & Storage Calculator
Estimate household water demand, size a tank for a period without replenishment, and explore rainfall and refill timing.
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Build a demand estimate
Add drinking, cooking, showers, hygiene, dishes, toilet flushing, laundry and cleaning. Include garden, animal and other water only when they draw from the same tank. Do not multiply a household total by household size again.
- Showers = people × showers per person per day × minutes per shower × litres per minute.
- Toilets = people × flushes per person per day × litres per flush. Enter zero for waterless toilets.
- Laundry = household loads per week × litres per load ÷ 7.
- Garden = square metres × millimetres per watering × waterings per week ÷ 7.
Weekly uses are daily averages. Example presets are editable assumptions, not measured household averages or health recommendations. Minimal Use includes shorter, less frequent showers and waterless toilets; Water Conscious and Higher Comfort use progressively larger activity assumptions. Presets affect household activities; outdoor demand and storage stay as entered.
Understand stored water
Nominal capacity is the tank’s total volume. Current fill includes water below the outlet and the protected reserve. Subtract these quantities once to find water available for normal use.
Let B be base daily use, s the safety allowance as a decimal, D = B × (1 + s), N the planning days, C nominal capacity, f the fill fraction, u the unusable fraction, R reserve in litres and L daily storage loss in litres.
Available water = max(0, C × f − C × u − R).
Full-tank capacity needed = ((D + L) × N + R) ÷ (1 − u).
Continuous autonomy = available water ÷ (D + L). When demand and loss are both zero, there is no depletion under these assumptions.
Worked example
Two people using 50 L each per day need 100 L/day before safety. A 10% allowance gives 110 L/day. For seven days with a 50 L reserve and 10% unusable capacity, required full-start capacity is (110 × 7 + 50) ÷ 0.9 = 911.11 L.
A 1,000 L tank at 80% fill has 800 − 100 − 50 = 650 L available, giving 650 ÷ 110 = 5.91 days of continuous autonomy with no storage loss or replenishment.
Rainfall and refills depend on timing
Rain yield in litres = roof area in m² × rainfall in mm × runoff coefficient × collection efficiency. For 100 m², 10 mm, 0.8 runoff and 0.9 efficiency, yield is 720 L before overflow.
The daily model starts with actual stored water, adds arrivals before demand, caps storage at tank capacity and records overflow. Household supply is served before storage loss, and both stop at the combined reserve and unusable floor. Unmet household demand and unserved storage loss are separate.
Evenly distributed rainfall is an optional estimate, not a forecast. Sufficient total rainfall can still leave shortages if it arrives late or overflows the tank. The capacity formula above covers a full tank with no replenishment; it does not establish rainfall reliability.
The interactive model supports 1–100 people, 1–366 planning days and up to 366 rainfall events. It rejects invalid active inputs rather than shortening a schedule. Inactive activities and supply settings are retained but do not affect results.
Frequently asked questions
How should I choose a reserve?
Enter a volume you want held back from everyday use, separately from water below the outlet. This tool does not determine the emergency reserve appropriate to your circumstances.
Is rainwater suitable for drinking?
This model tracks volumes only. Water quality, collection materials, treatment and suitability for each use require separate assessment. Drinking and cooking form a planning category; no water-quality approval or greywater credit is implied.
What should I measure?
Replace example assumptions with measured activity volumes, flows, duration and frequency. Use the water-per-load figure for your laundry equipment when available.
Conversions: 1 US liquid gallon = 3.785411784 L; 1 ft² = 0.09290304 m²; 1 inch = 25.4 mm. NIST Handbook 130. US gallons differ from Imperial gallons. This planning estimate does not establish guaranteed supply, water safety, compliance or engineering approval.
Turn your estimate into a practical tank plan
Measure a shower’s flow with a container and a timer, use appliance water-per-cycle figures where available, and record how often each activity happens. Keep per-person figures separate from whole-household totals. If a garden, animals or another building uses a different supply, model that supply separately.
Compare a normal-use plan with a lower-use plan and a longer period without replenishment. Choose your own reserve and safety allowance, then compare the calculated volume with available tank sizes, usable outlet volume and refill arrangements. The calculator does not select a commercial tank or establish rainfall reliability.
For rain collection, use roof catchment area and rainfall for the period being tested. A single large event can overflow the tank while later dry days still have unmet demand. Review the daily results as well as total yield.