Fairfax, VA rainwater: 34,630 gallons off a roof
Fairfax, VA: gallons off the roof each year
34,630gallons a year
That is a 1,500 sq ft asphalt roof under Fairfax, VA's normal 43.6 inches of rain, about 795 gallons for every inch that falls.
In Fairfax, VA a single barrel does most of the work. Storms come often enough that a small tank refills before it runs dry, and a bigger one would mostly sit full.
Rainfall: NOAA 1991-2020 climate normals, the Vienna station (2.7 mi away). Fairfax, VA ranks 30th wettest of the 39 cities covered in Virginia.
Month by month in Fairfax
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 3.28 | 2,607 | |
| Feb | 2.62 | 2,082 | |
| Mar | 3.84 | 3,052 | |
| Apr | 3.39 | 2,694 | 120 |
| May | 4.28 | 3,402 | 9 |
| Jun | 4.13 | 3,283 | 27 |
| Jul | 4.26 | 3,386 | 11 |
| Aug | 3.54 | 2,814 | 101 |
| Sep | 4.16 | 3,306 | 24 |
| Oct | 3.51 | 2,790 | 105 |
| Nov | 3.13 | 2,488 | |
| Dec | 3.43 | 2,726 |
May is the wettest month (4.28 in, 3,402 gal off the roof); Feb the driest (2.62 in). Garden shortfall is what a 200 sq ft vegetable bed needs beyond the rain that lands on it, April to October.
What size rain barrel for Fairfax
Best fit: one 55-gallon barrel, which delivers about 316 gallons to the garden in a normal year (79.7% of its need).
Before you buy, a 55-gallon barrel with a downspout diverter is the whole job. Check that the overflow points away from your foundation, because most of Fairfax's roof water will pass straight through it.
| Tank | Water used (gal/yr) | Spills over (gal/yr) |
|---|---|---|
| 55 gal | 316 | 33,144 |
| 110 gal | 316 | 33,144 |
| 165 gal | 316 | 33,144 |
| 275 gal | 316 | 33,144 |
| 550 gal | 316 | 33,144 |
| 1,000 gal | 316 | 33,144 |
| 1,500 gal | 316 | 33,144 |
| 2,500 gal | 316 | 33,144 |
| 5,000 gal | 316 | 33,144 |
Simulated storm by storm through a normal year using the station's count of days with 0.1 in or more of rain each month.
Is rainwater collection legal in Virginia?
Legal and promoted by the state. Va. Code 32.1-248.2 directs the Board of Health to set standards for rainwater harvesting systems, and the Department of Health with DEQ to promote rainwater use. No state storage cap; indoor/potable use must meet health regulations.
Checked against the current source on 2026-11-06. Source