Cistern Size Calculator
Roof and rainfall to cistern gallons or liters.
What it does
A roof is a catchment. Every inch of rain drops the same water per square foot, and a modest house can fill tanks fast. The question is how much of that sheet actually reaches the barrel.
The tool above multiplies roof footprint by rainfall and an efficiency share. Imperial mode reports gallons with the 0.623 factor; metric mode reports liters where millimeters times square meters convert directly. You also get a daily average and a tank count.
EPA WaterSense guides and extension rain harvest manuals teach the same product: area times depth times capture share. Texas A and M AgriLife and Georgia extension publish the 0.623 gallons per inch per square foot figure with first flush and gutter losses near 10 to 25 %.
Use harvest to pick tanks, not the reverse. Match storage to the dry weeks between rains, keep first flush and screens in the loop, and send overflow where it cannot find the foundation. For drinking, add treatment and check local rules.
How to use
- Pick imperial (ft2, inches, gallons) or metric (m2, mm, liters).
- Enter roof footprint area, not slope surface.
- Enter storm rainfall for the period.
- Set capture efficiency, 85 % for clean gutters.
- Read harvest, daily average, and tank count.
- Route overflow away from the house.
Formula
- roof
- Roof footprint in square meters or square feet.
- rain
- Storm rainfall in millimeters or inches.
- eff %
- Share captured after gutters and first flush.
- 0.623
- Gallons per inch per square foot.
One inch of rain on one square foot gives 0.623 gallons; one millimeter on one square meter gives one liter. Efficiency covers gutter loss, first flush, and filter waste. Daily average divides the storm by 30 for planning.
Worked example
Example: 1076 ft2 roof, 1 inch rain
1076 sq ft roof under 1 inch of rain at 85 % capture.
- Roof is 1076 ft2, rain is 1.0 in, efficiency 85 %.
- Harvest: 1076 x 1.0 x 0.623 x 0.85 = 569.8 gal.
- Daily: 569.8 / 30 = 19.0 gal per day.
- Tanks: 569.8 / 500 = 1.14 full 500 gal tanks.
- Plan: one 500 gal tank fills with a small surplus to overflow.
Reference table
| Storm | Gallons | 2000 L tanks |
|---|---|---|
| 0.5 in | 264.8 gal | 0.50 tanks |
| 1.0 in | 529.6 gal | 1.00 tanks |
| 1.5 in | 794.3 gal | 1.50 tanks |
| 2.0 in | 1,059.1 gal | 2.00 tanks |
| 3.0 in | 1,588.7 gal | 3.01 tanks |
Limits
- Estimates supply only. Demand, codes, and water quality set the final tank.
- Footprint area, not slope surface; steep roofs shed the same plan footprint.
- First flush and filter waste vary by storm.
- Potable use needs treatment and permits.
Common mistakes
- Using slope surface instead of footprint.
- Assuming 100 % capture with dirty gutters.
- Sizing for the record storm instead of the dry gap.
- Piping overflow toward the foundation.
- Drinking untreated roof runoff.
Try it yourself
Quick checks. Answers sit tucked under each question.
Harvest from 150 m2 and 40 mm at 80 %?
Harvest is 150 x 40 x 0.80 = 4,800 L. Daily is 160.0 L. Tanks are 2.4 x 2000 L.
Harvest from 1600 ft2 and 1.5 in at 80 %?
Harvest is 1600 x 1.5 x 0.623 x 0.80 = 1,196.2 gal. Daily is 39.9 gal. Tanks are 2.39 x 500 gal.
Questions
Where does 0.623 come from?
Roof area times rainfall times 0.623 gives gallons at full capture; multiply by efficiency for gutters and first flush loss. Metric is simpler: square meters times millimeters equals liters at full capture.
What efficiency should I use?
85 % suits clean gutters with a first flush diverter. Use 75 % with leaf load or long runs, 90 % with smooth metal and short downspouts. The tool lets you set it directly.
Should the tank hold the whole storm?
Size for the dry gap between storms, not the biggest storm. A tank that holds 2 to 4 weeks of use beats one that catches a rare deluge. Overflow must go safely away from foundations.
How do I keep stored water clean?
Screen inlets, dark tanks, and tight lids slow growth. First flush diverters drop the dirtiest wash. Treat or filter before drinking; rain can carry roof and air pollutants.
Can I connect a cistern to my house?
Yes where codes allow, often with permits past a size threshold. Some districts restrict potable tie in or require backflow guards. Check the local water office before plumbing it in.
What uses fit rainwater best?
Garden, laundry, and toilet use stretch mains the most. A 500 gal tank at 19 gal per day covers a month of garden beats in many climates.
Sources
- EPA WaterSense: outdoor water use
- Texas A and M AgriLife rainwater harvesting
- USGS water science: rainfall and runoff