Lakes & reservoirs

Acre-Feet and Percent Full: Reservoir Storage

By the Water Level Atlas team · Last reviewed Oct 21, 2026

Reservoir storage is usually measured in acre-feet. One acre-foot is enough water to cover one acre one foot deep: 43,560 cubic feet, or about 325,851 gallons (about 1,233 m³). "Percent full" compares the water stored now with a capacity, but agencies pick different capacities, so the same lake can be quoted with different percentages.

What is an acre-foot?

Picture a flat field of one acre (43,560 square feet) flooded to a depth of one foot. That volume is one acre-foot. USGS conversion tables give it as 43,560 cubic feet, 325,851 gallons, or 1,233 cubic meters.

Big reservoirs hold hundreds of thousands or millions of acre-feet, so agencies often write storage in thousands of acre-feet (KAF) or millions of acre-feet (MAF).

How does lake level relate to storage?

Gauges measure the elevation of the water surface, in feet. Storage is worked out from that elevation using an area-capacity table, also called a storage-elevation curve. The table lists, for each elevation, the lake's surface area and the total volume below that level.

Agencies build these tables by surveying the lake bed. For example, the Bureau of Reclamation surveyed Folsom Lake in 2005 with sonar and GPS for the underwater parts and aerial mapping for the rest, then computed a new table. The survey found that, at elevation 466.0 ft, the lake had a surface area of 11,140 acres and a total capacity of 966,823 acre-feet. It also measured about 43,407 acre-feet of capacity lost since 1955, mostly to sediment. Tables get updated over time, so a published capacity can change after a new survey.

Why do the top feet hold more water?

Most reservoirs sit in valleys that are narrow at the bottom and wide at the top. Each foot near the top covers a bigger area, so it holds far more water than a foot near the bottom.

Folsom Lake's 2005 table shows this clearly:

At Folsom, a foot near the top holds more than ten times as much water as an average foot in the inactive layer.

A handy rule of thumb follows from this: near full pool, one foot of change is roughly equal to the lake's surface area in acres, in acre-feet. Example (round numbers): a lake with a 10,000-acre surface near full pool holds very roughly 10,000 acre-feet in its top foot. This is only an approximation. The official area-capacity table is the real answer.

Why doesn't one foot lower mean the same percent at every lake?

Lakes have different shapes. The TVA Inspector General's review of TVA reservoir operations notes that "a five-foot drawdown on a shallow, broad surface lake would be different than a deep, narrow surface lake."

Example (round numbers): Lake A is broad and shallow. Its top 5 ft might hold 25% of its conservation storage. Lake B is a deep, narrow canyon lake. Its top 5 ft might hold only 5%. Both are "5 ft below full pool", but Lake A has lost far more of its usable water.

That is why feet below full pool and percent full tell you different things. Feet matter for docks, ramps and shorelines. Percent tells you about the water supply.

What does "percent of conservation pool" mean?

Many agencies measure percent full against the conservation pool, the working storage below full pool. The Texas Water Development Board, for example, says its percent-full figure "is based on Conservation Storage and Conservation Capacity and doesn't account for storage in flood pool."

This has two effects:

• A lake holding floodwater above full pool can still show 100%. The extra water is in the flood pool, which is not counted.
• Water below the conservation pool may not be counted either, depending on the agency.

What about "percent of total capacity"?

Some reports instead compare storage with the total capacity of the reservoir, which can include the inactive and dead layers and sometimes the flood pool. Percent of total capacity and percent of conservation pool can be quite different numbers for the same lake on the same day.

Before comparing two percentages, check:

1. Which capacity is the bottom of the fraction (conservation, active, or total)?
2. Which area-capacity table is being used (capacity changes after new surveys)?
3. Are both numbers from the same agency and the same date?

For the pools themselves, see what is full pool. For why the numbers move, see why lake levels change.

Using this on Water Level Atlas

Lake pages show storage only where the operator publishes it, and we label it with the agency's own units and definition. We do not convert elevation to storage ourselves or invent a capacity. Alongside storage you will see the current level, the change over 24 hours, 7 days and 30 days, and the normal range for the date. Our methodology page lists the source for each lake.