Rainwater Harvesting Tank Size Calculator

Rainwater Harvesting Guide

What do I need to know about Collection Calculation?

Annual harvest = roof area × annual rainfall × runoff coefficient. Runoff coefficient: tiles/slates ≈ 0.90 (small losses to wetting and splash). Flat felt roof ≈ 0.85. Green roof ≈ 0.50-0.65. Example: 60m² pitched tile roof in London (600mm annual rainfall): Harvest = 60 × 0.60 × 0.90 = 32.4m³ = 32,400 litres/year. This is a significant amount — equivalent to approximately 162 bathloads of water. However, distribution is uneven: summer rainfall often falls when demand for irrigation is highest.

What do I need to know about Demand and Tank Sizing?

Garden irrigation: approximately 15 litres/m² during dry summer months. An 80m² garden: 80 × 15 × 3 dry months = 3,600 litres peak demand. Toilet flushing: approximately 30 litres/person/day = 10,950 litres/year per person. Laundry: approximately 9,000-13,000 litres/year per household. Tank sizing rule: accommodate approximately 5-7% of annual demand as storage. For garden only use (3,600L peak): a 2,000-3,000L tank provides useful buffer. For toilet and laundry (40,000L demand): a 5,000-7,500L tank balances storage capacity against the space and cost of a larger installation — going much bigger gives diminishing returns since UK rainfall rarely lets a tank sit unused for long between refills, even in drier summer months.

What's the key thing to understand about Installation and Regulations?

Simple water butt: £20-80. Connects to a downpipe. Suitable for garden use. 200-500L capacity. Very simple DIY installation. Medium system (1,000-3,000L tank, submersible pump, overflow to sewer): £500-1,500 installed. Suitable for heavy garden use and car washing. Large underground system with whole-house non-potable supply: £3,000-8,000 professionally installed. Includes tank, pump, filtration, mains backup valve, separate pipework. Building Regulations: rainwater used inside the building requires a dedicated non-potable pipework system, clearly labelled (typically green) to avoid any risk of cross-connection with the drinking water supply, plus a mains backup with an air gap or approved backflow prevention device. Notify your water supplier before installing a system that could affect the mains connection, and use a WRAS-approved installer for anything feeding indoor appliances.

What do I need to know about Economics and Payback?

UK average water cost (metered): approximately £2.20 per m³ (combined water and sewage). Annual saving from 30m³ harvested: 30 × £2.20 = £66/year for garden-only use. For a simple £150 water butt setup: payback approximately 2-3 years. Large underground system (£5,000): saving £200-350/year from toilet and garden combined: payback 14-25 years — financially marginal. Non-financial benefits: reduced mains water demand. Reduced sewage system load during heavy rain (stormwater management). Resilience during hosepipe bans, since a rainwater tank keeps garden watering and car washing available when mains restrictions apply, is a benefit that doesn't show up in a pure payback calculation but matters to many households regardless of the financial case.

What's the key thing to understand about Sizing a Rainwater Harvesting System Correctly?

An effective rainwater harvesting system needs to be sized by balancing two independent factors: how much rain can realistically be collected (determined by roof catchment area and local average rainfall), and how much water will actually be used for the intended purpose (garden watering, toilet flushing, or other non-potable uses). A system sized only around available roof catchment, without considering actual demand, risks an oversized and unnecessarily expensive storage tank that's rarely filled to capacity between periods of use, while a system sized only around desired water usage without checking realistic collection potential risks a tank that runs dry during drier periods when it's needed most. A rough calculation multiplies roof area by local average rainfall and a collection efficiency factor (accounting for losses from evaporation, overflow during heavy downpours exceeding tank capacity, and the fact that a well-maintained guttering and filtration system rarely captures literally 100% of the rain falling on a roof) to estimate realistic annual collection potential, which can then be compared against expected usage to judge whether a given tank size and catchment area is well matched to the intended purpose. For garden-watering-only systems in temperate climates, a modest tank is often sufficient since demand is naturally lower during wetter periods when collection is highest and higher during drier periods when a larger buffer helps bridge gaps between rainfall events, whereas systems intended to supply toilet flushing or other continuous indoor uses typically need a larger tank and more careful year-round demand matching.

What should I know about Water Quality and Appropriate Uses for Harvested Rainwater?

Harvested rainwater is generally suitable for non-potable uses — garden watering, washing cars, outdoor cleaning, and (with appropriate filtration and a dedicated plumbed system) toilet flushing and washing machine use — but is not safe to drink without proper treatment and filtration specifically designed for potable water use, since rainwater collected from a roof can pick up contaminants from roofing materials, bird droppings, dust, and debris along the way. A basic first-flush diverter (which discards the initial, most contaminated run-off from a rainfall event before it enters the storage tank) and regular gutter and filter maintenance meaningfully improve the quality of water collected for non-potable uses, and are a worthwhile addition to most systems regardless of intended use. For any system intended to supply water indoors — toilet flushing or washing machines — proper plumbing separation from the mains potable water supply is essential, typically including clearly labelled pipework and backflow prevention, to ensure harvested rainwater can never accidentally mix with or contaminate the drinking water supply; this is both a practical safety matter and, in many jurisdictions, a specific building regulation requirement that needs professional installation and sign-off rather than a simple DIY plumbing connection. Checking local regulations and any requirements for backflow prevention and system labelling before connecting a rainwater system to indoor plumbing is an essential step that's easy to overlook when focused purely on the water-saving benefit.

What do I need to know about Roof Runoff Coefficients by Material?

Not all rain that falls on a roof makes it into the harvesting tank — some is lost to evaporation, splashing, and absorption into the roofing material itself. The runoff coefficient captures this loss as a simple multiplier between 0 and 1, and it varies noticeably by roofing material.

A green roof's low coefficient isn't a design flaw — it's deliberate, since the soil and vegetation layer is meant to retain and slowly release rainwater for its own benefit, the opposite goal from a conventional roof feeding a harvesting tank as efficiently as possible. A well-designed system also diverts the first flush of each rainfall event — the initial run-off that washes accumulated leaves, dust, and bird droppings off the roof — away from the storage tank before clean water is allowed through, which is one of the main reasons system efficiency sits noticeably below the roof's raw runoff coefficient.

What do I need to know about a Worked Example — Annual Collectable Rainwater Volume?

Because 1 mm of rainfall falling on 1 m² of roof equals exactly 1 litre of water, the full collection formula is: volume (litres) = roof area (m²) × annual rainfall (mm) × runoff coefficient × system efficiency (accounting for first-flush diversion and filter losses, often taken as roughly 0.85-0.9). Worked example: a house with an 80 m² pitched tile roof, in a UK region with a typical annual rainfall of 650 mm, using a runoff coefficient of 0.8 and a system efficiency of 0.9. Volume = 80 × 650 × 0.8 × 0.9 = 52,000 × 0.8 = 41,600; 41,600 × 0.9 = 37,440 litres/year (≈37.4 m³) That's a substantial volume — enough, at a typical outdoor-and-toilet usage rate of around 50 litres per day, to cover roughly two years of that demand from one year's collection, though in practice the tank is sized much smaller than the full annual yield because rainfall and demand are both spread unevenly across the year rather than arriving as a single lump sum. Sizing the tank correctly means balancing the roof's collection rate against realistic weekly or monthly demand and rainfall patterns, not just dividing the annual total by 365. Regional rainfall variation changes this calculation more than almost any other input: the same 80 m² roof in a drier eastern England location with closer to 550 mm of annual rainfall collects roughly 31,700 litres a year at the same 0.8 coefficient and 0.9 efficiency, while the same roof in a wetter western or Scottish location with 1,000 mm or more of annual rainfall could collect over 57,600 litres — which is why a system sized using a national average rainfall figure rather than local data can end up noticeably over- or under-sized for the specific property.