Rainwater catchment calculator

How this is calculated

0.623 is a unit conversion, not a loss factor. One inch of rain on one square foot is 1/12 of a cubic foot of water. A cubic foot holds 7.48 gallons, so 7.48 ÷ 12 = 0.623 gallons. You get the same number from the USGS figure of 27,154 gallons per inch of rain per acre: 27,154 ÷ 43,560 ft² = 0.623. The University of Arizona Cooperative Extension publication AZ1344, Harvesting Rainwater for Landscape Use, uses exactly this constant. Many online calculators describe 0.623 as already including spillage; it does not. The conversion assumes every drop is captured, which is why a separate efficiency term is needed.

Collection efficiency. AZ1344 Table 2 gives a runoff coefficient by surface: 0.90 to 0.95 for metal, gravel, asphalt shingle, fiberglass and mineral paper roofs, lower for rougher or absorbent surfaces. The calculator defaults to 0.90, the low end for a hard roof. The losses are real and specific: the first-flush diverter dumps the first few gallons of each storm, wind drives rain past the gutter, hard monsoon bursts overshoot a 5-inch gutter, water evaporates off a hot roof before it drains, and the tank overflows when a storm arrives full. If your gutters are undersized or you have no diverter, the real figure is lower than 0.90, not higher.

Roof footprint. Rain falls on the horizontal projection of the roof, so a 1,200 ft² footprint under a steep metal roof catches the same water as a 1,200 ft² footprint under a flat one. Measure the building's plan-view length and width and add any overhang that drains into the gutter. Do not add roof planes that drain off the back of the house unless you gutter them.

Monthly average. The calculator divides annual gallons by 12. In southern Arizona that number is useful for budgeting and useless for sizing storage, because the rain does not arrive monthly. See the Cochise County numbers below.

Cochise County defaults

Annual precipitation normals, 1991–2020, from NOAA's National Centers for Environmental Information:

• Rainfall: 12 in. The default is a fair valley-floor number for the Sulphur Springs and San Pedro valleys. Use the nearest station, and if you are up a bajada toward the Dragoons, Chiricahuas or Huachucas, expect more than the valley station reports.
• The monsoon is the year. At Pearce–Sunsites, July (2.58 in), August (2.86 in) and September (1.47 in) total 6.91 in, 57 percent of the annual normal. At Willcox the same three months total 6.31 in, 51 percent. April is the driest month at both stations (0.19 and 0.22 in). A tank sized from the monthly average will overflow in August and sit empty in May.
• Efficiency: 0.90. Most roofs here are metal or asphalt shingle, both in the 0.90 to 0.95 range in AZ1344. Keep 0.90 unless you have oversized gutters and no diverter losses to speak of.
• Collecting is legal. The University of Arizona Water Resources Research Center states that in Arizona it is legal to collect any rainwater that falls on your property for future use. Landscape, dust control and livestock use need little or no treatment; in-home use needs filtration and disinfection.

A worked example

A 30 × 40 ft cabin with a 1,200 ft² metal roof footprint at Pearce–Sunsites, 12.19 in/yr, efficiency 0.90. Gallons: 1,200 × 12.19 × 0.623 × 0.90 = 8,202 gal/yr, a monthly average of 683 gal. Running only the July–September rainfall (6.91 in) through the same roof gives 4,649 gal in three months, which is the quantity the tank has to be able to absorb. At 0.95 efficiency the annual figure is 8,658 gal; the 5-point efficiency difference is worth 456 gal a year, less than one monsoon storm.

For scale, 8,202 gal/yr is about 22 gal/day averaged across the year. That covers landscape and livestock use or a careful single occupant; it is not a household supply by itself at this roof size. The cistern sizing calculator works the storage side of the problem.

What this does not cover

• Storage sizing. Annual gallons tells you the supply; the tank has to hold the monsoon surplus and carry you through April. Monthly rainfall and your draw determine the tank, not this number.
• Dry years. Normals are 30-year averages. A year at 8 in happens, and a cistern plan built on 12 in has no margin in it.
• First-flush volume, gutter and downspout sizing, screen and inlet filtration, and overflow routing.
• Water quality and treatment. Roof material, bird load and dust change what comes off the roof. Potable use is a filtration and disinfection design, not a catchment calculation.
• Permits. Whether a large tank on a pad needs a county permit is a building-code question this page does not answer; ask Cochise County Development Services before you set one.
• Freeze. Valley-floor winters freeze exposed lines and fittings; see the related guide.

Sources

• University of Arizona Cooperative Extension, AZ1344 — Harvesting Rainwater for Landscape Use (PDF) — the 0.623 conversion and Table 2 runoff coefficients by roof material.
• NOAA National Centers for Environmental Information — U.S. Climate Normals, 1991–2020 — stations USC00026353 (Pearce–Sunsites), USC00029334 (Willcox), USW00093026 (Douglas–Bisbee International Airport), USC00027880 (Sierra Vista), USC00028619 (Tombstone).
• USGS Water Science School — Rain and Precipitation — 27,154 gallons per inch of rain per acre.
• University of Arizona Water Resources Research Center — Can I Harvest Rainwater? — legality of collection in Arizona and treatment for in-home use.

Related guides

• Hauled Water and Storage Tanks for Cochise County Off-Grid Land
• Hauled Water vs Drilling a Well: The Real Numbers
• Winter on the Valley Floor: Freeze Protection for Tanks, Pumps and Lines