Guide · Water units
cfs, gpm, acre-feet, and miner's inches: irrigation water units
Water rights are written in cfs or miner's inches, pumps are rated in gpm, and storage and seasonal duty are counted in acre-feet. Here is how they connect.
Short answer: 1 cfs = 448.8 gpm. One cfs running for 24 hours delivers 1.98 acre-feet. One acre-foot is 325,851 gallons. One cfs is almost exactly 1 acre-inch per hour. A miner's inch is 1/40 cfs in some states, 1/50 in others, and 1/38.4 in Colorado.
Flow rate: cfs and gpm
| From | To | Multiply by |
|---|---|---|
| cfs | gpm | 448.83 |
| gpm | cfs | 0.002228 |
| cfs | acre-feet per day | 1.9835 |
| cfs | acre-inches per hour | 0.9917 |
| gpm | acre-feet per day | 0.004419 |
Volume: acre-feet, acre-inches, gallons
| Unit | Equals |
|---|---|
| 1 acre-foot | 43,560 cubic feet = 325,851 gallons |
| 1 acre-inch | 3,630 cubic feet = 27,154 gallons |
| 1 cubic foot | 7.48 gallons |
The rule of thumb worth memorizing
1 cfs for 1 hour puts about 1 inch of water on 1 acre. From that, a set-time estimate is one line:
Example: applying a gross 4 inches to a 10-acre field with a 2 cfs head takes (10 × 4) / 2 = 20 hours.
Miner's inches by state
The miner's inch is a legal unit left over from hydraulic mining, and its size was set state by state.
| Miner's inches per cfs | One miner's inch | Where it is commonly used |
|---|---|---|
| 40 | 11.22 gpm | Arizona, California (statutory), Montana, Nevada, Oregon |
| 50 | 8.98 gpm | Idaho, Kansas, Nebraska, New Mexico, North Dakota, South Dakota, Utah, Washington; customary in southern California |
| 38.4 | 11.69 gpm | Colorado |
Always confirm the definition with your state water agency or on the water right itself. A right for "100 inches" is 2.5 cfs in Oregon but 2.0 cfs in Idaho — a 25 percent difference.
Turning a water right into a design flow
- Convert the right to cfs using your state's miner's inch if needed.
- Multiply by 448.83 for gpm if you are sizing a pump or pipe.
- Use cfs directly to size a ditch with Manning's equation or to check a headgate; use gpm for pipe sizing with Hazen-Williams.
Sources
- U.S. Geological Survey, water science glossary (miner's inch definitions by state).
- U.S. Bureau of Reclamation, Water Measurement Manual.
See every unit at once
FieldFlowCAD reports flow in gpm and cfs and volume in gallons and acre-feet side by side, straight from the drawing.