How to Choose the Right Rainwater Tank Size for Your Property
Choosing the right rainwater tank size is the most critical decision when installing a harvesting system. Too small, and you’ll run out of water during dry spells. Too large, and you’ve wasted money on unused capacity. This guide will help you calculate the perfect size for your needs.
The Three Factors That Determine Tank Size
Every property is unique, but all tank sizing calculations consider three core factors:
- Available Rainfall — How much water can you collect?
- Water Demand — How much water do you need?
- Available Space — Where will the tank go?
Let’s break down each factor.
Calculating Your Collection Potential
The fundamental equation is simple:
Roof Area (m²) × Annual Rainfall (mm) × Efficiency Factor = Annual Collection
For the UK:
- Southern England: ~600-700mm annual rainfall
- Midlands: ~700-800mm
- Northern England & Scotland: ~800-1,200mm
Efficiency Factor: Account for losses due to evaporation, splash-out, and first-flush diversion. Use 0.85 (85%) for most calculations.
Example Calculation:
- Semi-detached house in London
- Roof area: 60m²
- Annual rainfall: 650mm
- Collection: 60 × 650 × 0.85 = 33,150 litres per year
Understanding Your Water Demand
Different uses require vastly different amounts of water:
Garden Irrigation:
- Small garden (50m²): ~5,000L per growing season
- Medium garden (100m²): ~10,000L per growing season
- Large garden (200m²+): ~20,000L+ per growing season
Toilet Flushing (if plumbed in):
- Average person: 50-70 litres per day
- Family of four: 200-280 litres per day
Washing Machine:
- Average: 50 litres per load
- Family usage: ~350 litres per week
Car Washing:
- Pressure washer: ~100-200 litres per wash
- Hose: ~500+ litres per wash
Matching Supply to Demand
The goal is to size your tank so it rarely runs dry, but doesn’t stay full for months (wasted capacity).
The 2-4 Week Rule: For most UK properties, sizing your tank to hold 2-4 weeks of peak summer demand works well.
Example: A family wanting to irrigate a medium garden (10,000L per season) and wash their car monthly:
- Peak weekly demand: ~300L (garden) + ~150L (car) = 450L
- 4 weeks supply: 450 × 4 = 1,800L minimum
But remember — you’re collecting water seasonally. Your tank needs to store enough water captured during wet months to last through dry periods.
Seasonal Storage Requirements
UK rainfall is highly seasonal:
- Winter (Dec-Feb): 40-50% of annual rainfall
- Spring (Mar-May): 20-25%
- Summer (Jun-Aug): 15-20%
- Autumn (Sep-Nov): 20-25%
Your tank needs enough capacity to capture winter/spring surplus and store it for summer use.
Rule of Thumb for Gardens: Size your tank to hold 25-33% of your annual irrigation needs. This typically covers the 2-3 driest months when mains water would otherwise be needed.
Tank Sizing by Property Type
Small Terraced House (roof ~40m²):
- Garden irrigation only: 1,000-2,000L
- With toilet flushing: 2,000-3,000L
- Recommended: 2,500L underground or 2×1000L water butts
Semi-Detached House (roof ~60m²):
- Garden irrigation only: 2,000-3,000L
- With toilet flushing: 3,000-5,000L
- Recommended: 5,000L underground or 3×1000L linked butts
Detached House (roof ~100m²):
- Garden irrigation only: 3,000-5,000L
- With toilet flushing: 5,000-7,000L
- Recommended: 7,000L underground tank
Large Property with Pool/Garden (roof 150m²+):
- Significant irrigation needs: 10,000L+
- Recommended: 10,000-15,000L underground tank
Commercial/Agricultural:
- Calculations must consider daily demand and collection area
- Typical range: 20,000L to 100,000L+
- Professional assessment strongly recommended
Above Ground vs Underground Considerations
Above-Ground Tanks:
- Cheaper to install
- Easier to access for maintenance
- Limited by garden space and aesthetics
- More vulnerable to freezing
- Typical sizes: 200L to 5,000L
Underground Tanks:
- No visual impact
- Frost protection
- Larger capacities economical
- Higher installation cost
- Typical sizes: 2,000L to 20,000L+
The Overflow Question
Every tank needs overflow management. Size your overflow pipe to handle your region’s maximum rainfall intensity (not just average).
In the UK, design for 50-75mm/hour rainfall events. A 100m² roof collecting at this rate generates ~6,000 litres/hour — your overflow must handle this.
Direct overflow to:
- Permeable areas of your garden
- Rain gardens or swales
- Soakaways (subject to ground conditions)
- Never directly to neighbours’ properties or public drains without permission
Space Requirements
Above-Ground Tank Space:
- 1,000L round tank: ~1.2m diameter × 1.8m height
- 2,500L round tank: ~1.5m diameter × 2.0m height
- 5,000L round tank: ~2.0m diameter × 2.2m height
Underground Tank Excavation:
- Add 0.5m clearance on all sides
- Access manhole needs clear space above
- Consider delivery access for large tanks
When to Go Bigger
Consider upsizing your tank if:
- You’re in a drier region (southern England, east coast)
- Your garden is particularly water-hungry (large lawns, vegetables)
- You plan to add toilet flushing or other indoor uses later
- You want resilience against extended dry periods
- Space and budget allow it
The cost difference between a 3,000L and 5,000L underground tank is often surprisingly small relative to total installation costs.
When to Go Smaller
Downsize considerations:
- You’re in a high-rainfall area (Scotland, Lake District)
- Minimal garden irrigation needs
- Budget constraints (above-ground options are cheaper)
- Very limited space
- You plan to move house within 2-3 years
Professional Sizing Service
While these guidelines work for most situations, complex properties benefit from professional assessment:
- Large roofs with multiple collection areas
- Properties with both irrigation and indoor use
- Commercial or agricultural applications
- Sites with unusual rainfall patterns (coastal, high altitude)
- Properties with space constraints requiring creative solutions
At Rainwater Doctor, we use detailed calculations considering your specific roof area, local rainfall data, soil conditions, and water usage patterns. We’ll model different tank sizes against historical rainfall data to find your optimal capacity.
Quick Reference Sizing Table
| Property Type | Roof Area | Primary Use | Recommended Tank Size |
|---|---|---|---|
| Flat/Apartment | 20m² | Window washing, small balcony | 200-500L |
| Terraced House | 40m² | Garden irrigation | 1,000-2,000L |
| Semi-Detached | 60m² | Garden + some indoor | 3,000-5,000L |
| Detached House | 100m² | Full garden + indoor | 5,000-7,000L |
| Large Property | 150m²+ | Extensive irrigation + indoor | 10,000L+ |
| Small Business | 200m²+ | Toilets, yard washing | 10,000-20,000L |
| Farm/Agricultural | 500m²+ | Livestock, irrigation | 20,000L+ |
Get Your Personalised Sizing
Every property is different. Factors like roof orientation, shading, local microclimate, and your specific water usage patterns all affect the ideal tank size.
Contact Rainwater Doctor for a free site assessment. We’ll calculate your collection potential, understand your water needs, and recommend the perfect tank size for your property and budget.
Don’t guess on tank size — get it right the first time with professional guidance.