From tank to garden

Rain barrel watering calculator.

How long will your water last? Check your stored water, then compare a MET Norway refill forecast with no new rain.

Illustration: rain from a roof refills the tank; garden watering draws stored water on dry days.

Tank capacity (L) Starting fill (%)

Keep a reserve

Keep in reserve (% of tank capacity)

The default keeps 10% of capacity in reserve; it is an editable planning allowance. Include water you cannot draw from below the outlet or need to leave for the pump. Use a measured daily amount where possible, or estimate garden demand.

Daily water use (L/day) Days to cover

Use your daily amount, or adjust an example.

Gravity or pump? Optional

Measure vertically from the emitters to the water surface, now and at your planned refill level. Enter a negative height when the water surface is below the emitters.

Water surface above emitters now (m) Water surface at refill level (m) Kit minimum pressure (bar)

Hoses, filters, valves and flowing emitters reduce pressure. Match the refill height to the reserve you plan to keep; this tool cannot convert volume to height without your tank geometry.

Have a delivery measurement? Optional

Measured total delivery (L/min)

Use water actually delivered by the whole setup at its refill level. Collect output from all emitters for a known time, add it together, and divide by minutes. The tank tap's unrestricted flow is not the drip system's delivery.

Roof & collection For refill

Connected roof footprint (m²)

Quick estimates, not measured averages. Use only the horizontal roof area draining into this tank.

Rain collected after losses (%)

These settings affect the refill illustration and forecast. The water budget credits no new rain.

Your water budget

Watering covered

Available after reserve

Steady entered use; no new rainfall or top-up credited. This is a water-volume check, not confirmation of plant needs or equipment reliability.

Higher use & time away

Next-rain refill forecast Optional · about 5 days

Uses your stored water, daily use, reserve and Roof & collection settings above.

Town or postal code

Collection & forecast assumptions

80% is an editable allowance for filters, first flush and other losses. One millimetre of rain on one square metre gives one litre before those losses.

The projection adds collection, spills anything above capacity, then applies daily use evenly by hour, drawing only above your reserve. Rain is spread evenly within each one- or six-hour forecast interval; the actual timing can differ.

Starting fill applies at the chart’s start, the next complete forecast hour. Snow, sleet and precipitation at temperatures of 1°C or below are excluded from refill, as are periods without temperature data. Frozen stored water, thawing and frozen plumbing are not modelled.

Forecasts can change. The comparison uses complete available intervals up to five days; its actual end is shown. See the forecast method and limits.

Projected water level

Explore the water flow

Roof collection fills the tank, garden use draws stored water, and excess leaves through the overflow outlet. — Edit: Roof · Fill level · Water use

Water-level values

Stored water at daily checkpoints
Time	Forecast rain	No new rain

Find watering equipment

Check the kit’s stated minimum pressure and test its total delivery at your refill level. A supplier link does not confirm compatibility.

Delivery country

Example: how long does a 55 US gallon rain barrel last?

A full 55 US gallon barrel (about 208.2 L), keeping 10% in reserve, has 49.5 US gallons (187.4 L) available. At an entered use of 5 US gallons (18.9 L) per day, it covers 9 full watering days. At 25% more use, it covers 7 full days. Neither case credits new rain.

To cover seven days while retaining that reserve, start at least 73.7% full at the entered use, or 89.6% full at the higher use. These are illustrative water-budget inputs; they do not establish what your plants need or that the kit can deliver it.

For context, University of Minnesota Extension describes 50–55 US gallon home rain barrels. The daily use and reserve here are example assumptions.

Will the next rain refill my rain barrel?

The refill chart compares forecast rain with no new rain, using the same starting water, daily use and reserve. Make a water check above, then open Next-rain refill forecast to choose a place and the roof area connected to your barrel or tank. MET Norway supplies the weather estimate for up to five days.

For an illustrative 10 mm shower on 30 m² of connected roof at 80% collection, potential inflow is 240 L (about 63.4 US gallons) before tank overflow. How much remains depends on your starting fill, available space and water use. This example is arithmetic, not a local weather forecast.

The chart excludes snow and freezing-risk precipitation from refill. Rain can change or miss your roof; retain a backup where a water shortfall matters. See the forecast method and limits.

Method, limits & a test before you leave

Copy water-budget link includes the assumptions in your completed water check, including entered pressure and delivery measurements. The optional forecast location and roof inputs are not included. Anyone with the link can reopen the budget; the values travel in the URL fragment rather than being saved by this site. Review the fill level and other inputs when returning to a saved check.

Usable water = capacity × (starting fill − reserve) ÷ 100, floored at zero. Full watering days = usable water ÷ daily use, rounded down. The high-use check uses 25% more daily water; it is an illustrative sensitivity case.

The starting fill needed is reserve (%) + 100 × daily use × days ÷ capacity. If it exceeds 100%, the required capacity of a full tank is daily use × days ÷ (1 − reserve ÷ 100). A 100% reserve cannot supply any water. These are volume requirements for the entered period, not tank purchase recommendations or refill forecasts.

Gravity pressure comes from water-surface height above the emitter: about 0.0981 bar per metre, or 0.4335 psi per foot, for fresh water. Tank capacity does not create pressure. The static ceiling excludes hose, filter, fitting, valve and emitter losses. A sufficient ceiling is a reason to test, not a pass for the installation.

The height needed for a stated positive minimum pressure is that pressure in bar ÷ 0.0980665. It is only the theoretical minimum before losses. The refill-height shortfall is the difference between that minimum and your entered refill height, floored at zero. Displayed minimum fill, capacity and height requirements round up.

The base water budget credits no new rain. The optional refill chart uses MET Norway’s weather forecast separately; it does not change the budget or the historical tank-size comparison. This helper does not size a pump, certify kit compatibility, predict plant water demand, assess water quality or design a stand. Use the actual equipment specifications and a physical delivery test. A low-water height that fails may require refilling earlier than the volume budget suggests.

Test the complete kit while you are home, including the intended timer.
Test at your planned refill level and collect from the furthest emitter as well as the whole setup.
Check that each plant receives the intended water and the pump remains within its specified operating limits.
For time away, arrange a refill or fallback where the entered budget is insufficient.

Sources: Drip Depot: gravity head and falling water level; NMSU: low-pressure drip irrigation research.

Compare tank sizes with historical rainfall →