Example Data Table
| Case |
Input Data |
Formula Type |
Approximate Gross Capacity |
| Rectangular farm pond |
80 m length, 45 m width, 5 m depth |
Length × Width × Depth |
18,000 m³ |
| Sloped reservoir basin |
Bottom 60 × 30 m, Top 90 × 55 m, Depth 6 m |
Frustum formula |
19,470 m³ |
| Round storage tank |
32 m diameter, 4.5 m water depth |
π × r² × h |
3,619 m³ |
| Contour survey |
Elevation-area rows from field survey |
Trapezoidal area integration |
Depends on survey intervals |
Formula Used
Rectangular reservoir: V = L × W × D. This works when the reservoir has nearly vertical sides or a consistent plan area.
Trapezoidal reservoir: V = h ÷ 3 × (A₁ + A₂ + √(A₁ × A₂)). This estimates a basin with sloped sides. A₁ is bottom area. A₂ is top water surface area.
Cylindrical reservoir: V = π × r² × h. This fits circular tanks and round reservoirs.
Contour method: V = Σ [(Aᵢ + Aᵢ₊₁) ÷ 2 × ΔE]. This is useful when survey elevations and water surface areas are available.
Net usable capacity equals gross capacity minus sediment reserve, dead storage, and seasonal losses. The calculator converts every result into common planning units.
How to Use This Calculator
Select the method that best matches your reservoir shape. Use the rectangular method for simple basins. Use the trapezoidal method for sloped sides. Use the cylindrical method for round storage. Use the contour method for surveyed reservoirs.
Choose the length unit and area unit. Enter dimensions only for the selected method. Add freeboard if the reservoir should not be filled to the physical top. Add sediment, dead storage, and seasonal loss percentages when planning usable water.
Enter daily demand if you want the calculator to estimate coverage days. Press Calculate to show the result above the form. Use the CSV or PDF button to save the report.
Reservoir Capacity Planning Guide
Why total capacity matters
Total reservoir capacity is the full water storage available at a chosen water level. It supports irrigation planning, livestock supply, storm storage, fire reserves, and industrial use. A small error can cause shortage during dry periods. It can also increase construction cost when the basin is oversized.
Choose the right method
The best method depends on the reservoir shape. A rectangular method is fast for lined ponds or tanks. A cylindrical method fits round tanks. A trapezoidal frustum method is better for earthwork basins with sloped banks. The contour method is best for natural reservoirs because it uses survey areas at several elevations.
Plan for unusable storage
Gross capacity does not always equal useful capacity. Sediment can reduce storage over time. Dead storage may remain below the outlet. Evaporation and seepage can reduce seasonal water volume. This calculator lets you deduct these allowances. The net result gives a more practical planning value.
Check units carefully
Reservoir projects often mix meters, feet, acres, liters, gallons, and acre-feet. Unit mistakes can create serious design errors. Enter dimensions using one length unit. Enter contour areas using the selected area unit. The calculator converts results into several volume units for easy reporting.
Use results wisely
This tool gives an engineering estimate. Final design should consider soil conditions, side slopes, spillway capacity, seepage control, legal limits, and field survey accuracy. For important projects, compare the result with professional survey data. Review freeboard and outlet levels before approving a storage plan.
FAQs
1. What is total reservoir capacity?
Total reservoir capacity is the gross water volume stored up to a chosen water level. It includes active water, dead storage, and reserves before deductions.
2. Which method should I choose?
Choose rectangular for simple basins, cylindrical for round tanks, trapezoidal for sloped reservoirs, and contour for surveyed elevation-area data.
3. What does freeboard mean?
Freeboard is the vertical distance kept between the maximum water level and the reservoir top. It protects against waves, storms, and overtopping.
4. Why include sediment reserve?
Sediment slowly occupies storage space. A sediment reserve keeps the plan realistic by setting aside volume that may become unavailable over time.
5. What is dead storage?
Dead storage is water below the outlet or pump intake. It remains in the reservoir but may not be usable for normal supply.
6. How does the contour method work?
It sorts elevation-area rows and sums each layer. Each layer uses the average of two adjacent surface areas multiplied by elevation difference.
7. Can I calculate coverage days?
Yes. Enter daily demand and its unit. The calculator divides net usable capacity by daily demand to estimate supply duration.
8. Are the results final design values?
No. Results are planning estimates. Critical reservoirs should be checked with field survey data, hydraulic design, soil review, and local rules.