Calculator
Example data table
| Example | Method | Key inputs | Settings | Output (approx.) |
|---|---|---|---|---|
| 1 | Rectangular | 6 m × 3 m × 1.2 m | Freeboard 0.1 m, Dead 0.2 m, Level 100% | Total 19.8 m³ (19,800 L), Live 16.2 m³ |
| 2 | Circular | Dia 2.5 m, Depth 1.5 m | Freeboard 0.15 m, Dead 0 m, Level 100% | Total 6.63 m³ (6,627 L), Live 6.63 m³ |
| 3 | Sloped rectangular | Top 8×5, Bottom 6×3, Depth 2 m | Freeboard 0.2 m, Dead 0 m, Level 100% | Total 48.92 m³ (48,924 L), Live 48.92 m³ |
Formula used
- Rectangular: V = L × W × h
- Circular: V = π × (D/2)² × h
- Elliptical: V = π × (A/2) × (B/2) × h
- Sloped sides (frustum): prismoidal rule V = (h/6) × (A₁ + 4Aₘ + A₂), where areas are taken at bottom, mid-depth, and top (or waterline).
- Irregular estimate: V = Area × AvgDepth × ShapeFactor
How to use this calculator
- Select the shape that best matches your reservoir.
- Choose one length unit and enter all dimensions consistently.
- Enter total depth (to rim), then the needed dimensions.
- Set freeboard to keep water below the rim.
- Add dead depth if your intake sits above the bottom.
- Adjust water level to represent partial filling.
- Use reserve percentage for losses or safety margin.
- Press Calculate, then download CSV or PDF reports.
Garden reservoir sizing notes
Daily demand and refill planning
Start with an estimate of watering and top-up demand in liters per day. Example: 40 drip emitters rated 4 L/hour running 30 minutes use about 80 L/day. If your supply can reliably refill 200 L/day, then a 1,000 L reservoir offers roughly five days of buffer during interruptions and heat spikes.
Freeboard, storm allowance, and overflow
Freeboard is the safety gap between the rim and your target waterline. It reduces usable depth but helps prevent overtopping from splash, wind setup, and sudden rainfall. In many small garden tanks and lined basins, 50–150 mm is practical. Pair storage with a defined overflow route to protect edges, liners, and nearby paving.
Live storage vs dead storage for reliability
Dead storage represents the lower layer you prefer not to draw because of sediment, debris, or intake elevation. Keeping 100–300 mm as dead depth can reduce clogging and extend pump life. The calculator reports both dead and live volumes so you can size usable storage for irrigation schedules, not just total capacity.
Selecting the best geometry for the build
Use straight-sided methods for masonry reservoirs and manufactured tanks. For excavated ponds, sloped-side options often fit better because side slopes expand surface area toward the rim. If your outline is irregular, measure surface area at the waterline and apply a shape factor: 0.7–0.9 suits gentle slopes, while values closer to 1.0 fit near-vertical walls.
Conversions help procurement and operation. The tool reports m³ and liters for planning, plus gallons for pump catalog comparisons. As a quick check, 1 m³ equals 1,000 L and about 264 US gallons. Side slopes also increase wet area and evaporation, so consider shade, windbreaks, and overflow routing. Export CSV/PDF results to document assumptions, compare options, and share a clear record with installers or maintenance staff.
FAQs
1) What shape should I choose?
Choose the closest match: rectangular for box tanks, circular for round tanks, sloped options for excavations, elliptical for pond-like basins, and irregular when you only know surface area and average depth.
2) Why does freeboard reduce the result?
Freeboard lowers the filled depth below the rim. Because volume scales with depth, a small freeboard can reduce capacity noticeably, especially for reservoirs with large surface area at the waterline.
3) What is dead depth used for?
Dead depth estimates water you do not intend to use due to sediment, debris, or intake height. The calculator treats this as unusable volume and reports live storage separately.
4) How accurate is the irregular method?
It is a planning estimate. Accuracy depends on surface-area measurement and how well the shape factor represents side slopes. Use it to compare options, then confirm with field measurements when possible.
5) Can I calculate volume for a partially filled reservoir?
Yes. Set the water level percentage to represent current fill after freeboard. The tool calculates volume at that waterline and updates live/dead storage accordingly.
6) Which unit is best for pump sizing?
Use liters or gallons to compare pump flow rates and estimate run time. Use cubic meters for excavation, liner discussions, and overall capacity checks in metric-based project notes.
7) Does this include evaporation losses?
Evaporation depends on climate, shade, wind, and surface area. Use the reserve/loss percentage to add a buffer, then adjust that value after observing real water level changes over time.