The Complete Math Guide to Garden Irrigation

The Three Numbers Every Irrigation Plan Needs

• Three numbers drive every irrigation plan: coverage area, flow rate, and target water depth.
• Converting inches of water to gallons uses one constant: 0.62 gallons per square foot per inch of water.
• A 100 square foot bed needing 1 inch of water per week requires 62 gallons, regardless of delivery method.
• Runtime differs by system type because flow rate differs: drip, soaker hose, and sprinklers each need their own runtime calculation.
• Rechecking actual output with a simple catch test corrects for real-world losses the formulas alone can't predict.

Irrigation planning looks complicated because it involves three different measurements — area, depth, and flow rate — that most gardeners have never had to combine. In practice, it's one formula applied three different ways depending on your delivery system, as the numbers above show. Once you know the formula, drip lines, soaker hoses, and sprinklers all become variations on the same calculation.

The Universal Conversion: Inches to Gallons

Vegetable gardens are usually recommended 1 to 1.5 inches of water per week. To turn that into a number your hose or system can act on, use this constant: 1 inch of water over 1 square foot equals 0.62 gallons. Multiply your bed's square footage by the inches needed, then by 0.62, to get total gallons.

Worked example: a 100 square foot bed needing 1 inch weekly: 100 times 1 times 0.62, or 62 gallons per week. The same bed in a hot spell needing 1.5 inches: 100 times 1.5 times 0.62, or 93 gallons per week. This gallon figure is your target regardless of whether you deliver it through drip, soaker hose, or sprinkler — only the runtime to reach it changes.

Runtime for Drip Systems

Drip runtime equals target gallons divided by total emitter flow rate. If your 100 square foot bed has 20 emitters at 1 GPH each, total flow rate is 20 GPH. Runtime for 62 gallons is 62 divided by 20, or 3.1 hours weekly — split across 2-3 sessions of about an hour to an hour and a half each, so water has time to soak in rather than run off.

Runtime for Soaker Hoses

Soaker hoses are typically rated by length rather than a per-emitter GPH, commonly delivering about 0.5 to 1 gallon per foot per hour depending on water pressure and hose age. A 50-foot soaker hose at 0.6 gallons per foot per hour delivers 50 times 0.6, or 30 gallons per hour. For our 62-gallon weekly target, runtime is 62 divided by 30, or about 2.1 hours weekly, split into two 1-hour sessions.

Runtime for Sprinklers

Sprinklers are rated in inches per hour rather than gallons, which actually simplifies things: since your target is already in inches (1 to 1.5 inches weekly), runtime is just target inches divided by the sprinkler's inches-per-hour rate. A common oscillating sprinkler puts down about 0.5 inches per hour. For a 1-inch weekly target: 1 divided by 0.5, or 2 hours weekly, typically split into two 1-hour sessions to reduce runoff on compacted soil.

Accounting for Coverage Overlap and Gaps

Sprinkler math assumes even coverage, which real sprinklers rarely provide on their own — output is usually heavier near the sprinkler head and lighter at the pattern's edge. The fix is head-to-head spacing: position sprinklers so each one's spray reaches the next sprinkler's head, not just its own coverage circle's edge. This overlap, typically pushing actual coverage per sprinkler down by 20 to 30 percent versus the manufacturer's max-radius claim, is why real sprinkler layouts need more heads than a simple radius calculation suggests.

Verifying With a Catch Test

All three formulas above are starting points, not guarantees, because real flow rates drift from their rated numbers due to pressure, age, and clogging. Place several small containers throughout the coverage area, run the system for exactly 15 minutes, and measure. If your system should deliver 62 gallons over 3.1 hours, a 15-minute catch test across the bed should show roughly one-twelfth of that per container's share — adjust your runtime up or down based on what you actually measure, not just the rated numbers.

Every irrigation system, regardless of type, answers to the same underlying question: how many gallons does this bed need, and how fast can this system deliver them. Converting inches to gallons with the 0.62 constant gives you the target; each system's own flow rate gives you the runtime. Check the real output once a season and the math stays accurate all year.