# Rainwater Harvest Calculator Estimate collected rain volume, first flush loss and remaining tank space. [Open this tool](https://whatson.pk/gardening-rain-harvest-calculator/) Category: Gardening Rain falling on a catchment is not all stored in the tank. Collection losses and first flush reduce the available volume. Enter rainfall, catchment area and collection assumptions, then compare the remaining tank space. ## Inputs and controls * Horizontal roof catchment (m²). Allowed range: 0.01 to 1000000 * Event rain depth (mm). Allowed range: 0 to 10000 * Runoff collection share (%). Allowed range: 0 to 100 * First flush volume (L). Allowed range: 0 to 100000000 * Tank capacity (L). Allowed range: 0.1 to 100000000 * Water already in tank (L). Allowed range: 0 to 100000000 ## How to use * Enter horizontal roof catchment, event rain depth, runoff collection share and first flush volume. * Enter tank capacity and water already in tank. * Select Calculate result. Read the returned values alongside the assumptions below before copying, exporting or acting on the result. ## Why use this tool Collection and overflow are separated, useful when checking whether a larger tank could capture more of the event. ## Before relying on the result Collection arithmetic does not assess water quality. Overflow is water that does not fit after the first flush. ## Tool limitations Collection arithmetic does not assess water quality. Overflow is water that does not fit after the first flush. ## Questions ### Does collected volume mean potable water? No. The calculation does not assess water quality or treatment needs. ### What should I check before using Rainwater Harvest Calculator? Check Horizontal roof catchment, Event rain depth, Runoff collection share and First flush volume. Use the exact units shown beside the controls and replace any example values with your own inputs. Source page last modified (UTC): 2026-10-04T13:08:12+00:00 This profile describes the tool. It does not execute it or supply a professional certification.