Cistern Capacity Calculator Water Storage
Calculate water volume in rectangular, cylindrical, and oval cisterns | US Gallons | Imperial Gallons | Liters | Cubic Feet
What is a Cistern?
A cistern is a waterproof receptacle for holding water, typically used for rainwater harvesting, fire suppression, or backup water storage. This calculator helps determine the capacity of rectangular, cylindrical, and oval cisterns with support for feet+inches input, multiple unit outputs (US Gallons, Imperial Gallons, Liters, Cubic Feet), fill/empty time, and rainwater catchment estimation.
Rectangular Cistern Dimensions (Feet + Inches)
Fill / Empty Time Calculator
Rainwater Catchment Calculator
Cistern Capacity & Analysis Results
Fill / Empty Time Estimates
Rainwater Catchment Potential
Dimensions & Volume Breakdown
Usage Recommendations
Cistern Capacity Calculator – Quick Guide
Do you really need this calculator? Yes. Getting cistern size wrong costs money. Too small = you run out of water. Too big = you waste cash. This calculator gives you the right number in seconds.
How to Start (Even if You're New)
You don't need experience. Just pick your tank shape, type in the measurements, and click calculate. The math happens automatically. You'll see gallons, liters, fill time, and how much rain you can collect.
Why Bother Learning This?
- Save money – buy the right size the first time
- Stop guessing – know exactly how much water you have
- Avoid running dry during droughts
- Skip costly installation mistakes
Expert Tips That Actually Help
- Add 20% extra – dry spells happen. Future needs change.
- Measure inside – tank walls take up space.
- Expect sediment – dirt builds up at the bottom. Plan to clean it.
- Check local rules – some cities require minimum water storage.
Cistern Shapes & Simple Formulas
| Shape | Formula |
|---|---|
| Rectangular | Length × Width × Height |
| Cylindrical (horizontal) | π × r² × Length |
| Cylindrical (vertical) | π × r² × Height |
| Spherical | (4/3) × π × r³ |
Don't worry – the calculator does the math for you.
Final Thought (Short & Real)
You don't need to master anything. Just measure twice, click once, and build with confidence. That's it.
Frequently Asked Questions
What does this calculator actually do?
It tells you how many gallons or liters your tank holds. No more guessing.
How long to learn this?
About 2 minutes for basic shapes. A few more minutes for odd-shaped tanks.
What mistakes should I avoid?
- Skipping inside measurements
- Forgetting to convert inches to feet (calculator does this for you)
- Ignoring dead space at the bottom
Where can I learn more?
Check rainwater harvesting blogs, YouTube tutorials, or your state's water management website.
Use it. It's free. It saves money. It stops you from buying the wrong tank.
A 1,000-gallon cistern on a 500 sq ft roof can collect ~1,600 gallons per year in a rainy area. That's real water.
Measure inside. Add 20% buffer. Use the calculator. Done.
One Costly Mistake to Avoid
Getting cistern size wrong isn't just a math error – it's a headache. Too small = no water when you need it. Too big = wasted money. This calculator fixes both.
Why Accurate Capacity Matters
I've seen people run dry during droughts because their tank was too small. That's why I built this. Homeowners, farmers, and engineers use it to get the right size – the first time.
Whether you collect rain, store backup water, or irrigate a garden – knowing exact gallons changes everything. This tool also estimates fill time and rain you can catch.
How to Use (Step by Step)
- Pick shape: Rectangular, Cylindrical, or Oval
- Enter feet + inches (calculator converts for you)
- Optional: Add pump flow rate for fill time
- Optional: Add roof area + rainfall for rain capture
- Click Calculate → See results instantly
Pro Tips (Do These)
- Add 20% buffer – dry periods and future needs
- Measure inside – tank walls steal space
- Plan for sediment – dirt builds up over time
- Check codes – some areas require minimum storage
IMPORTANT (but simple): This is an estimate. Your real tank may be slightly different due to manufacturing or sediment. Always double‑check with your tank supplier.
⚠️ Note for pros: This calculator uses standard assumptions. Your actual project may vary. Verify with your material supplier.
— Calculate with confidence, MultiTooSite team
More FAQs
How do I convert feet and inches to decimal feet?
Decimal feet = Feet + (Inches ÷ 12). Example: 5 ft 6 in = 5.5 ft. But the calculator does this for you automatically.
US gallons vs Imperial gallons – what's the difference?
US gallon = 3.785 liters
Imperial gallon (UK) = 4.546 liters
Imperial is ~20% bigger. This calculator shows both.
How is rain catchment calculated?
Roof area (sq ft) × yearly rain (inches) × 0.623 × efficiency.
Example: 1,000 sq ft roof, 1 inch of rain = ~623 gallons at 100% efficiency. Real life is about 85% due to loss.
What flow rate should I use for fill time?
- City water: 5–15 GPM
- Well pump: 5–10 GPM
- Rain gravity feed: 2–5 GPM
Use your actual pump number for best results.
