Reservoir Volume Calculator

Enter your reservoir's length, width, average depth, and basin shape to calculate volume in acre-feet, gallons, cubic feet, cubic meters, and liters.

Created by Luis GonzalezLast updated: June 17, 2026

How to Use This Calculator

1
Enter Reservoir Dimensions
Input the length and width (in feet) of the reservoir. For oval basins, use the major and minor axes. Then enter the average water depth — not the maximum — by averaging several measurements across shallow and deep areas.
2
Select Shape and Calculate
Choose the basin shape (Rectangular, Elliptical/Oval, or Triangular) to apply the correct area factor. Click Calculate to see volume in acre-feet, gallons, and cubic feet, plus a water-level scenario table and actionable insights.

Example Calculation

A rancher in central Texas needs to verify that their newly excavated stock pond holds enough water for a full growing season of irrigation.

Length (ft)

250

Width (ft)

140

Average Depth (ft)

Basin Shape

Rectangular

Results

Volume

9.64 acre-ft

Gallons

3,141,818 gal

Surface Area

0.80 acres

Cubic Feet

420,000 ft³

Depth Assessment

Moderate depth

Tips

Average Multiple Depth Readings

A 1-foot depth error on a 1-acre pond (43,560 sq ft) translates to 43,560 cubic feet — over 325,851 gallons of miscalculated water. Take at least 5 depth measurements across the basin and average them for reliable volume estimates.

Factor in Annual Evaporation Losses

A reservoir with 0.80 acres of surface area in a moderate climate (4 ft/year evaporation) loses roughly 1,047,273 gallons annually — about 33.3% of a 3.14-million-gallon pool. Deeper, narrower designs minimize surface-to-volume ratio and reduce percentage losses.

Budget for Excavation Costs

Earthen reservoir excavation typically runs $2-$8 per cubic yard in 2026. A 420,000 cubic-foot reservoir equals about 15,556 cubic yards, putting estimated dig costs at $31,111-$124,444 before liner or grading.

Plan for Sedimentation

Sediment inflow reduces usable depth over time. USDA estimates 0.5-2 inches of sediment per year in agricultural watersheds. Re-survey depth every 3-5 years and budget for periodic dredging to maintain capacity.

Reservoir Volume Calculator: Accurate Capacity Estimates for 2026

The Reservoir Volume Calculator converts length, width, average depth, and basin shape into precise volume figures in acre-feet, gallons, cubic feet, and cubic meters. Whether you are sizing a stock pond, planning irrigation storage, or evaluating a municipal impoundment, accurate volume data is the foundation of water-resource management. For example, a rectangular basin measuring 250 ft by 140 ft with a 12 ft average depth holds 9.64 acre-feet — roughly 3.14 million gallons — a critical benchmark for agricultural water planning in 2026.

How the Volume Formula Works

The calculator uses a straightforward geometric approach:

Surface Area (sq ft) = Length x Width x Shape Factor
Volume (cubic feet)  = Surface Area x Average Depth

Conversion	Factor
Cubic feet to acre-feet	/ 43,560
Cubic feet to gallons	x 7.48052
Cubic feet to liters	x 28.3168
Cubic feet to cubic meters	x 0.0283168

Shape factors: Rectangular = 1.0, Elliptical = 0.7854 (pi/4), Triangular = 0.5. Choosing the correct shape prevents errors of 20-50% in your estimate.

💡 A 1-foot error in average depth on a 1-acre pond adds or subtracts 43,560 cubic feet — over 325,851 gallons. Always average at least 5 depth measurements across the basin.

Worked Example: Sizing a Farm Pond

Consider a rectangular irrigation pond:

Length: 250 ft | Width: 140 ft | Average Depth: 12 ft | Shape: Rectangular

Surface Area = 250 x 140 x 1.0 = 35,000 sq ft (0.80 acres)
Volume = 35,000 x 12 = 420,000 ft³
Acre-feet = 420,000 / 43,560 = 9.64 acre-ft
Gallons = 420,000 x 7.48052 = 3,141,818 gal

At 9.64 acre-feet, this pond can irrigate roughly 9-10 acres of cropland for a growing season in a moderate climate. In the arid Southwest, plan for 2-3 acre-feet per acre.

💡 Pair reservoir volume with crop-specific demand using our Water Use per Acre Calculator to match storage capacity to actual irrigation needs.

Evaporation, Sedimentation, and Long-Term Management

Two factors steadily erode reservoir capacity:

Evaporation removes 2-8 feet of water depth per year depending on climate. A 0.80-acre surface in a moderate zone (4 ft/year) loses about 1,047,273 gallons annually — 33.3% of a full 3.14-million-gallon reservoir. Deeper, narrower designs minimize the surface-to-volume ratio and cut percentage losses.

Sedimentation fills the basin from the bottom. Agricultural watersheds add 0.5-2 inches of sediment per year (USDA estimates). Over 10 years, that can reduce a 12-foot reservoir to an effective depth of 10-11 feet, cutting volume by up to 14%. Re-survey depth every 3-5 years and budget for periodic dredging.

Factor	Annual Impact (250 x 140 ft pond, 12 ft depth)
Evaporation (4 ft/yr)	~1,047,273 gal lost (33.3% of capacity)
Sedimentation (1 in/yr)	~21,818 gal capacity lost per year
Combined 10-year loss	Up to 40-50% of original volume without maintenance

Reservoir Construction Costs in 2026

Earthen reservoir excavation typically runs $2-$8 per cubic yard. For the example pond (420,000 ft³ = 15,556 yd³), excavation alone costs an estimated $31,111-$124,444. Add a synthetic liner at $0.50-$1.50 per square foot if your soil percolation rate exceeds 2 inches per hour. Include grading, inlet/outlet structures, and permitting — which can double the total project budget. Get a geotechnical survey before breaking ground to confirm soil suitability and identify any clay or rock layers.

💡 Our Biodiversity Score Calculator can help evaluate the ecological impact of a new reservoir on surrounding habitats and guide mitigation planning.

Frequently Asked Questions

What is an acre-foot and why is it used for reservoir volume?

An acre-foot is the volume needed to cover one acre of land one foot deep — equal to 43,560 cubic feet or 325,851 gallons. It is the standard unit for water rights, irrigation districts, and municipal supply planning because it scales neatly to real-world acreage, making it easy to compare storage capacity against crop water demand.

How does basin shape change the volume calculation?

Shape determines the surface-area multiplier applied to length times width. A rectangular basin uses a factor of 1.0 (full area). An elliptical or oval basin uses pi/4 (approximately 0.7854), reducing volume to about 78.5% of rectangular. A triangular basin uses 0.5, cutting volume in half. Choosing the wrong shape can cause a 20-50% error in your estimate.

How do I accurately measure average depth?

Take depth readings at 5 or more evenly spaced points across the basin — including the deepest and shallowest zones — and compute the arithmetic mean. Using only the maximum depth will overestimate volume, sometimes by 30-50% for irregularly shaped ponds.

How much water does evaporation remove each year?

Annual evaporation varies by climate: 2-3 feet in the northern US, 4-5 feet in the central US, and 6-8 feet in the arid Southwest. For a reservoir with 0.80 acres of surface and 4 ft/year evaporation, that is roughly 1,047,273 gallons lost annually. Deeper reservoirs lose a smaller percentage of total volume.

Can this calculator handle metric inputs?

The calculator accepts dimensions in feet. To convert metric measurements, multiply meters by 3.28084 to get feet before entering them. The results include cubic meters and liters alongside US customary units for easy metric reference.

How much does it cost to build a reservoir in 2026?

Earthen excavation typically costs $2-$8 per cubic yard. A 420,000-cubic-foot pond (about 15,556 cubic yards) runs roughly $31,000-$124,000 for excavation alone. Add $0.50-$1.50 per square foot for a synthetic liner, plus grading, compaction, and permitting fees, which can double the total project cost.