Watering & Irrigation Calculators

Despite the UK's reputation for rain, summer drought is an increasing reality for British gardeners. Average UK rainfall of 885mm per year sounds generous, but much of it falls outside the growing season, and climate change is making dry spells longer and more intense — particularly in southern and eastern England, which already receives less than 600mm annually. Our water butt size calculator helps you capture as much roof runoff as possible, sizing your tank based on roof area, local rainfall data, and expected usage. A typical semi-detached house roof can collect over 45,000 litres per year.

For established gardens, efficient irrigation systems save both water and time. Drip irrigation is one of the most water-efficient methods, delivering water directly to plant roots with minimal evaporation. Our drip irrigation calculator works out dripper spacing, pipe lengths, and total flow rates for your layout. The irrigation timer calculator helps programme your system for early morning or evening watering, when evaporation losses are lowest. For lawns, the sprinkler coverage calculator ensures even distribution by calculating head placement and overlap patterns for pop-up or oscillating sprinklers.

Rainwater harvesting is increasingly popular with environmentally conscious UK gardeners, and our rainwater harvesting calculator shows just how much free water your roof can provide across the year. For smaller gardens or those without an irrigation system, the watering can calculator provides a practical estimate of how many trips you need to make to properly water beds and containers. The evapotranspiration calculator uses temperature, humidity, and wind speed to estimate daily water loss from your garden — particularly useful during June, July, and August when UK gardens lose 3-5mm of moisture per day through evaporation and plant transpiration.