Rainwater Harvesting Worksheet
Measure the footprint, not the slope. One inch on one square foot is 0.6234 gallons - exactly.
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The arithmetic
- Catchment
- footprint length x width
- ⚠ Footprint, NOT sloped area
- rain falls vertically
- 1 in on 1 sq ft
- 0.6234 gal (144 ÷ 231, exact)
- 1 mm on 1 m²
- 1 litre exactly
- Water off the roof
- area x rain x 0.6234 x efficiency
- Overflow
- collected − storage
- 1 in on 1,000 sq ft
- about 623 gal before losses
- 1 US gallon
- 231 cubic inches, by definition
Your setup
- Building
- Footprint length
- Footprint width
- CATCHMENT AREA
- Typical monthly rainfall
- Measured efficiency
- Barrel capacity
- Number of barrels
- TOTAL STORAGE
- Weekly garden use
- WEEKS OF WATERING
Measure your own efficiency
| Date | Rainfall | Expected gal | Actually caught | Efficiency % |
|---|---|---|---|---|
Before and after installing
- Footprint measured at ground level, overhang included
- Local rules on rainwater capture checked
- Overflow route decided and sized for the FULL flow
- Overflow discharging away from the foundation and off neighbours' land
- Barrel on a level base rated for the filled weight
- Sealed lid with a screened inlet against mosquitoes and debris
- First-flush diverter fitted if the roof carries much debris
- Tap high enough to get a watering can under
- Winter plan - drained or frost-protected as needed
- Garden water only, unless properly treated
Water is heavy — check what it is standing on
A US gallon of water weighs about 8.34 lb, so a full 55-gallon barrel is roughly 460 lb plus the barrel. That needs a level, solid base. Raising it on blocks for tap clearance is normal, but the stack has to be rated for the filled weight, not the empty one - and it is the filled one that falls on somebody.
Size storage to your USE, not your roof
Catching everything a roof sheds in a storm takes a startling number of barrels and is rarely the point. The useful question is how many weeks of watering you want held between rains. Work back from your weekly use and the gap you want to cover, and the storage figure becomes reasonable.