Why this conversion matters for turf drainage

Designing artificial turf that plays great and drains right depends on matching rainfall intensity to outlet capacity. Rain data is often in inches per hour, while pumps, pipes, and inlets are rated in gallons per minute. Converting correctly keeps fields playable, landscapes clean, and projects on budget.

The math behind the units

  • Start with 1 square foot of area.
  • Apply a 1 inch depth of water. That is 144 cubic inches of volume.
  • Convert to gallons. 1 gallon is 231 cubic inches, so 144 cubic inches is 0.623 gallons.
  • Spread that hourly volume across minutes. 0.623 gallons per hour divided by 60 minutes equals 0.01038 gallons per minute for each inch per hour over one square foot.

Round 0.01038 to 0.0104 for practical design. Scale linearly with your rainfall rate and your total square footage.

Worked examples

Example 1: Drainage rating to system flow

A turf and base assembly is rated at 20 inches per hour. Per square foot, the corresponding flow is 20 × 0.0104 = 0.208 gpm per sq ft.

  • For 1,000 sq ft: 0.208 × 1,000 = 208 gpm total.
  • For 5,000 sq ft: 0.208 × 5,000 = 1,040 gpm total.

Example 2: Known outlet to equivalent rainfall

A trench drain and outlet can move 300 gpm from a 2,500 sq ft putting green. The equivalent rainfall intensity is 300 gpm ÷ 2,500 sq ft ÷ 0.0104 = 11.54 inches per hour.

Example 3: Small area detail

A 200 sq ft landing needs to handle a 10 in/hr cloudburst. Per square foot flow is 10 × 0.0104 = 0.104 gpm. Total is 0.104 × 200 = 20.8 gpm.

Design tips that keep you honest

  • Use a safety factor. Start with 1.2 to 2.0 to cover clogging, seam lines, and real-world variability.
  • Check the bottleneck. Subbase permeability, geotextiles, pipe inverts, and outlet restrictions can limit flow more than the turf perforations.
  • Size from local rain data. NOAA Atlas 14 intensities are commonly specified in inches per hour for the chosen storm and duration.
  • Mind the slope and spread. Flat zones collect water. Crowned or sloped profiles move it to inlets faster.
  • Coordinate details. Shock pads, infills, and edge restraints must maintain flow paths and not trap water.

Quick reference pairs

  • 1 in/hr = 0.0104 gpm per sq ft
  • 5 in/hr = 0.0520 gpm per sq ft
  • 10 in/hr = 0.104 gpm per sq ft
  • 20 in/hr = 0.208 gpm per sq ft
  • 40 in/hr = 0.416 gpm per sq ft

Put it to work

Bring your area, targeted storm intensity, and outlet constraints. Convert, size, and verify with a clean safety factor. If you want a second set of eyes, FusionTurf will review your numbers and help you dial in a buildable, reliable plan.