Cesspit Calculator

Cesspit Calculator — estimate cesspit for your project. Fast, accurate, free. No signup required.

Drainage and storage sizing connect rainfall, flow, and construction volume. The Pipe Flow Rate calculator checks conveyance capacity, the Construction Material calculator estimates excavation or fill quantities, and the Building Regulations calculator provides a companion compliance check.

The water sizing calculator performs drainage design, water storage, and plumbing sizing calculations for construction projects — from calculating the peak stormwater runoff from a roof or driveway to sizing a septic tank, soakaway, rainwater harvesting tank, swimming pool, or irrigation system. Water management is one of the most regulated aspects of building control: inadequate drainage leads to flooding, structural damage, and public health risks; oversized systems waste money; and incorrect soakaway or septic tank sizing can cause enforcement action and contamination. UK Building Regulations Approved Document H (Drainage and Waste Disposal) sets minimum requirements for all drainage systems.

The core calculation types are: peak flow (using the Rational Method Q = CiA for surface drainage), volume (length × width × depth for tanks and ponds), and infiltration rate (for soakaways). All sizing should be conservative — drainage engineers design for the 1-in-10-year or 1-in-100-year storm event depending on consequence of flooding.

The mode is selected automatically based on your topic: pipe/drainage (peak flow calculation), rainwater harvesting (annual yield), or tank/volume (pool, pond, septic, cesspit).
For drainage: enter the catchment area (m²), design storm intensity (typically 50 mm/hr for a 1-in-10-year event in most UK locations), and the runoff coefficient for your surface type.
For rainwater harvesting: enter roof collection area (m²), local annual rainfall (mm), and collection efficiency (0.75–0.90 for a clean roof).
For tanks: enter length, width, and depth to calculate volume in m³ and litres.
Compare results against Building Regulations Approved Document H minimums and BS EN standards for your application.

Water sizing formulas

Peak flow (Rational Method): Q = C × i × A / 360 where C = runoff coefficient (0–1), i = rainfall intensity (mm/hr), A = catchment area (m²); Q in litres/second.

Minimum pipe diameter: d = √(4Q / π × v) where v = pipe flow velocity (typically 1.0 m/s for gravity drainage); result in metres, then select next standard pipe size (100, 150, 225, 300 mm).

Rainwater yield: V = A × (R/1000) × η where A = roof area (m²), R = annual rainfall (mm), η = collection efficiency (0.75–0.90).

Tank volume: V = L × W × D (m³). For septic tanks: BS 6297 requires minimum 2,700 litres for up to 4 persons, plus 180 litres per additional person.

Example: 100m² driveway, 50mm/hr storm, runoff coefficient 0.9. Q = 0.9 × 50 × 100 / 360 = 12.5 l/s. At 1m/s velocity: pipe area = 0.0125 m² → diameter = 126mm → use 150mm pipe.

Reading your water sizing result

Standard sizes and compliance thresholds

Minimum pipe sizes (Building Regulations Approved Document H): foul water branch pipes = 75mm minimum; soil stacks = 100mm minimum; surface water = 100mm minimum for individual properties. Standard gravity drainage pipes: 100mm (residential), 150mm (shared drains, gullies), 225mm, 300mm (main sewers). Soakaway minimum volume: 1m³ per 15m² of impermeable area drained (guidance only — always perform a percolation test per BRE Digest 365). Septic tank minimum (BS 6297): 2.7m³ (2,700 litres) for up to 4 persons, +0.18m³ per additional person. Rainwater harvesting system sizing: WRAS Guidance Document recommends tank capacity of approximately 5% of annual roof yield or 2 weeks' toilet-flushing demand, whichever is greater.

Construction tips and best practices

For soakaway design, always conduct a percolation test (BRE Digest 365 / BS EN ISO 22282) first — soil infiltration rate varies by 10–100× across short distances and cannot be estimated from soil type alone.
Use a 1-in-100-year storm event (plus 40% climate change allowance for post-2024 designs) for any drainage system where flooding consequences are significant (adjacent to buildings, basements, or watercourses).
Swimming pool volume determines heating, chemical dosing, and pump sizing — a 12m × 5m × 1.5m pool holds 90m³ (90,000 litres), requiring a pump circulating at least 15m³/hr for good turnover.
Rainwater harvesting tanks should include a 100mm freeboard above the maximum water level, an overflow pipe discharging to an appropriate outfall, and a first-flush diverter to prevent contamination from roof debris.
Septic tanks must be at least 5m from any dwelling, 10m from any watercourse, and 50m from any abstraction point or borehole under Environment Agency Regulations.

The UK loses approximately 3 billion litres of water per day to leakage in water mains — about 23% of total water supply.
A standard UK domestic property generates approximately 150 litres per person per day of foul wastewater — a 3-bedroom house with 4 occupants produces 600 litres per day needing treatment or drainage.
A 50mm/hr rainfall intensity on a 200m² roof produces 10,000 litres (10m³) per hour — equivalent to emptying a full bath every 2 minutes.
An Olympic swimming pool holds 2,500m³ (2.5 million litres) — the residential domestic swimming pool average is 45–90m³.

Common mistakes to avoid

Using average rainfall instead of design storm intensity — drainage systems must cope with peak rainfall rates, not averages. A 1-in-10-year storm in southern England can exceed 50mm/hr for a 15-minute duration.
Ignoring upstream catchment area — a soakaway or drain serving only a back garden must still be sized for all impermeable surfaces that drain to it, including roofs, paths, and driveways.
Sizing a septic tank for current occupancy only — always size for the maximum likely occupancy of the property (number of bedrooms + 1 as a rule of thumb).
Connecting surface water to the foul sewer — it is illegal in England and Wales to connect surface water drainage to the public foul sewer. Always check connection arrangements with the sewerage undertaker.

All drainage works in England and Wales are subject to Building Regulations Approved Document H (Drainage and Waste Disposal). Connections to public sewers require approval from the sewerage undertaker (e.g. Thames Water, Severn Trent). Septic tanks and soakaways must comply with the Environment Agency's Regulatory Position Statement RPS 063 and, where applicable, require a permit under the Environmental Permitting (England and Wales) Regulations 2016. Swimming pools and water features must comply with health and safety legislation regarding water safety and, where they are publicly accessible, the Swimming Pools and Water Recreation legislation. This calculator provides preliminary sizing estimates only and does not constitute a drainage design or regulatory approval.