If you're considering building your own rainwater harvesting system but you're unsure what size system you need, I've created a few rainwater harvesting calculators that will help you.
Rainwater Harvesting Calculator
Calculate the amount of rainwater you can collect from your roof based on how large the surface area of your roof is and how much rainfall you get.
If you do not know how much rainfall you get, I added the Monthly Rainfall Stats for the US and Monthly Rainfall Stats for the UK below.
For an annual estimate, I also added annual rainfall statistics for the UK and US.
Drinking Water Calculator
Calculate the estimated drinking water needs for your family.
Family Water Needs Calculator
Calculate the estimated water needs for your family. This is based on the average daily consumption of 50 gallons per day; assuming everyone in the family cooks, cleans and washes everything they need.
Realistically-speaking though, you could probably survive on less.
Average Monthly Rainfall Statistics (United States) in Inches
| State | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
| Alabama | 4.66 | 4.61 | 5.27 | 4.67 | 4.3 | 5.29 | 5.56 | 4.65 | 4.05 | 3.13 | 4.29 | 4.92 |
| Alaska | 4 | 3.45 | 3.15 | 2.26 | 2.01 | 1.75 | 2.12 | 2.63 | 4.13 | 5.08 | 5.04 | 4.6 |
| Arizona | 0.99 | 1.05 | 1.18 | 0.51 | 0.26 | 0.16 | 1.13 | 1.36 | 0.74 | 0.64 | 0.71 | 1.01 |
| Arkansas | 4.15 | 4.24 | 4.82 | 4.29 | 4.8 | 4.56 | 3.67 | 3.06 | 3.39 | 3.7 | 4.06 | 4.75 |
| California | 3.23 | 3.17 | 2.56 | 1.18 | 0.43 | 0.09 | 0.02 | 0.05 | 0.23 | 0.77 | 1.68 | 2.77 |
| Colorado | 0.47 | 0.59 | 1.19 | 1.91 | 2.54 | 1.81 | 2.2 | 1.84 | 1.42 | 0.99 | 0.63 | 0.46 |
| Connecticut | 3.98 | 3.24 | 4.41 | 4.6 | 4.14 | 4.1 | 4.36 | 4.08 | 4.03 | 3.56 | 3.49 | 4.15 |
| Delaware | 3.55 | 2.87 | 4.06 | 3.61 | 3.82 | 3.43 | 4.21 | 3.73 | 3.46 | 3.23 | 3 | 3.51 |
| Florida | 3.15 | 2.84 | 3.51 | 3.05 | 4.37 | 7.34 | 7.59 | 7.38 | 6.05 | 3.78 | 3.09 | 2.88 |
| Georgia | 4.72 | 4.47 | 5.35 | 3.86 | 3.49 | 4.08 | 5.01 | 4.31 | 3.47 | 3.14 | 4.21 | 4.57 |
| Hawaii | 6.16 | 5.08 | 5.03 | 4.32 | 3.34 | 2.31 | 1.94 | 2.34 | 3.04 | 4.56 | 5.97 | 6.92 |
| Idaho | 1.55 | 1.23 | 1.59 | 1.7 | 2.12 | 1.61 | 0.69 | 0.77 | 0.86 | 1.23 | 1.54 | 1.48 |
| Illinois | 2.32 | 2.19 | 3.06 | 3.88 | 4.23 | 4.17 | 4.07 | 3.65 | 3.13 | 2.69 | 2.97 | 2.63 |
| Indiana | 2.68 | 2.39 | 3.33 | 4.09 | 4.52 | 4.13 | 4.06 | 3.47 | 2.87 | 2.55 | 2.92 | 2.78 |
| Iowa | 1.07 | 0.96 | 2.07 | 3.14 | 3.93 | 4.21 | 3.61 | 3.22 | 2.62 | 2.11 | 1.73 | 1.22 |
| Kansas | 0.91 | 1.14 | 2.1 | 3.06 | 3.61 | 3.29 | 3.53 | 2.94 | 2.31 | 1.96 | 1.31 | 1.04 |
| Kentucky | 3.34 | 3.23 | 4.22 | 4.28 | 4.85 | 4.41 | 4.02 | 3.68 | 3.09 | 2.53 | 3.19 | 3.53 |
| Louisiana | 5.03 | 4.51 | 5.08 | 4.57 | 5.19 | 6.89 | 6.99 | 5.97 | 5.13 | 3.77 | 4.52 | 4.9 |
| Maine | 3.69 | 3.18 | 3.8 | 3.75 | 3.77 | 3.31 | 3.21 | 3.35 | 3.58 | 3.45 | 3.83 | 3.74 |
| Maryland | 3.23 | 2.87 | 3.72 | 3.61 | 3.76 | 3.28 | 3.97 | 3.68 | 3.24 | 3.01 | 2.91 | 3.14 |
| Massachusetts | 3.68 | 3.19 | 3.95 | 3.99 | 3.83 | 3.48 | 3.71 | 3.5 | 3.46 | 3.27 | 3.39 | 3.69 |
| Michigan | 2.08 | 1.84 | 2.53 | 3.03 | 3.39 | 3.4 | 3.19 | 3.2 | 2.81 | 2.26 | 2.26 | 2.2 |
| Minnesota | 0.94 | 0.89 | 1.82 | 2.51 | 3.2 | 3.76 | 3.98 | 3.32 | 2.69 | 2.04 | 1.33 | 1.03 |
| Mississippi | 4.91 | 4.79 | 5.51 | 5.27 | 4.92 | 5.25 | 5.82 | 4.88 | 4 | 3.26 | 4.15 | 4.86 |
| Missouri | 2.21 | 2.39 | 3.61 | 4.15 | 4.7 | 4.01 | 3.85 | 3.46 | 2.87 | 2.55 | 2.82 | 2.47 |
| Montana | 0.74 | 0.76 | 1.48 | 2.06 | 2.78 | 2.14 | 1.31 | 1.01 | 0.88 | 0.91 | 0.7 | 0.66 |
| Nebraska | 0.63 | 0.74 | 1.75 | 2.49 | 3.42 | 3.45 | 3.01 | 2.58 | 1.91 | 1.29 | 0.91 | 0.72 |
| Nevada | 0.7 | 0.71 | 0.81 | 0.47 | 0.53 | 0.4 | 0.47 | 0.49 | 0.29 | 0.35 | 0.57 | 0.76 |
| New Hampshire | 3.66 | 3.17 | 3.88 | 4.19 | 4.12 | 3.83 | 3.92 | 3.78 | 3.75 | 3.5 | 3.56 | 3.61 |
| New Jersey | 3.53 | 3.14 | 4.14 | 4.14 | 4.15 | 3.82 | 4.26 | 4.03 | 4.11 | 3.6 | 3.65 | 3.76 |
| New Mexico | 0.42 | 0.49 | 0.6 | 0.57 | 0.92 | 1.71 | 2.09 | 2.15 | 1.46 | 0.93 | 0.6 | 0.42 |
| New York | 2.94 | 2.59 | 3.44 | 3.68 | 3.77 | 3.64 | 3.78 | 3.53 | 3.52 | 3.21 | 3.21 | 3.14 |
| North Carolina | 3.61 | 3.71 | 4.4 | 3.76 | 4.1 | 3.78 | 4.62 | 4.63 | 4.32 | 3.56 | 3.32 | 3.69 |
| North Dakota | 0.51 | 0.58 | 1.14 | 1.73 | 2.42 | 2.67 | 2.38 | 1.94 | 1.48 | 0.98 | 0.68 | 0.56 |
| Ohio | 2.71 | 2.43 | 3.36 | 3.88 | 4.17 | 3.88 | 4.03 | 3.49 | 3.07 | 2.65 | 2.86 | 2.77 |
| Oklahoma | 1.53 | 1.79 | 2.68 | 3.21 | 4.05 | 3.93 | 2.78 | 2.44 | 2.43 | 2.24 | 1.99 | 1.68 |
| Oregon | 4.54 | 4.08 | 3.75 | 2.88 | 2.21 | 1.63 | 0.77 | 0.86 | 1.38 | 2.61 | 4.01 | 4.87 |
| Pennsylvania | 3.03 | 2.63 | 3.78 | 3.97 | 4.06 | 3.94 | 4.34 | 4.09 | 3.97 | 3.44 | 3.14 | 3.07 |
| Rhode Island | 3.73 | 3.31 | 4.23 | 4.15 | 4.07 | 3.91 | 4.13 | 4.06 | 4.04 | 3.77 | 3.64 | 3.8 |
| South Carolina | 3.51 | 3.77 | 4.45 | 3.44 | 3.47 | 3.82 | 5.59 | 5.75 | 4.43 | 3.44 | 3.36 | 3.17 |
| South Dakota | 0.48 | 0.61 | 1.47 | 2.08 | 2.85 | 3.21 | 2.73 | 2.21 | 1.58 | 1.05 | 0.74 | 0.57 |
| Tennessee | 3.88 | 3.99 | 4.87 | 4.57 | 4.71 | 4.41 | 4.11 | 3.77 | 3.06 | 2.59 | 3.45 | 3.9 |
| Texas | 1.8 | 2.05 | 2.41 | 2.51 | 3.48 | 3.44 | 2.63 | 2.5 | 2.59 | 2.37 | 1.93 | 1.94 |
| Utah | 1.48 | 1.54 | 1.66 | 1.47 | 1.52 | 0.94 | 0.6 | 0.76 | 0.97 | 1.36 | 1.47 | 1.43 |
| Vermont | 2.86 | 2.37 | 3.15 | 3.57 | 3.8 | 3.62 | 3.93 | 3.85 | 3.19 | 2.96 | 3.02 | 2.89 |
| Virginia | 3.22 | 3.13 | 3.92 | 3.54 | 3.64 | 3.45 | 4.1 | 3.96 | 3.57 | 3.07 | 2.97 | 3.11 |
| Washington | 5.2 | 4.14 | 3.87 | 2.54 | 1.93 | 1.1 | 0.59 | 0.7 | 1.26 | 2.73 | 4.6 | 5.24 |
| West Virginia | 3.15 | 2.96 | 4.1 | 3.83 | 4.31 | 4.03 | 4.37 | 3.9 | 3.15 | 2.68 | 2.9 | 3.01 |
| Wisconsin | 1.49 | 1.3 | 2.27 | 3.26 | 3.69 | 4.22 | 4.1 | 3.61 | 2.73 | 2.24 | 1.92 | 1.63 |
| Wyoming | 0.54 | 0.58 | 1.3 | 1.83 | 2.24 | 2.22 | 1.57 | 1.24 | 0.88 | 0.78 | 0.55 | 0.54 |
Average Monthly Rainfall Statistics UK in Inches
| Region | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
| Scotland | 5.75 | 4.32 | 4.54 | 3.23 | 3.27 | 2.86 | 3.48 | 3.66 | 3.94 | 5.39 | 5.51 | 5.3 |
| Northern Ireland | 3.61 | 2.64 | 2.95 | 2.52 | 2.6 | 2.66 | 2.87 | 3.05 | 2.95 | 3.66 | 3.71 | 3.62 |
| North West England | 3.99 | 3.13 | 3.41 | 2.61 | 2.75 | 2.87 | 2.99 | 3.33 | 2.95 | 3.83 | 3.95 | 3.95 |
| North East England | 2.56 | 1.83 | 1.94 | 1.79 | 2.04 | 2.33 | 2.43 | 2.59 | 2.37 | 3.17 | 2.99 | 2.62 |
| Midlands | 2.34 | 1.71 | 1.86 | 1.75 | 1.94 | 2.07 | 2.26 | 2.3 | 2.08 | 2.63 | 2.69 | 2.58 |
| East of England | 2.03 | 1.47 | 1.64 | 1.59 | 1.79 | 1.8 | 1.74 | 1.94 | 2.01 | 2.39 | 2.42 | 2.27 |
| South West England | 3.74 | 2.92 | 3.33 | 2.48 | 2.53 | 2.25 | 2.42 | 2.7 | 2.66 | 3.41 | 3.65 | 3.71 |
| South East England | 2.16 | 1.61 | 1.77 | 1.71 | 1.96 | 1.88 | 1.81 | 2.05 | 2.12 | 2.51 | 2.47 | 2.35 |
| Wales | 4.03 | 3.28 | 3.46 | 2.91 | 3.04 | 3.03 | 2.95 | 3.19 | 2.99 | 3.94 | 4.11 | 3.97 |
| London | 2 | 1.43 | 1.52 | 1.54 | 1.88 | 1.77 | 1.72 | 1.92 | 2.08 | 2.37 | 2.43 | 2.31 |
Annual Rainfall Statistics For Every State In The US (in inches)
| State | Annual Average (in inches) |
| Alabama | 58.3 |
| Alaska | 22.6 |
| Arizona | 13.59 |
| Arkansas | 49.76 |
| California | 21.19 |
| Colorado | 15.94 |
| Connecticut | 48.42 |
| Delaware | 45.71 |
| Florida | 54.48 |
| Georgia | 50.67 |
| Hawaii | 63.7 |
| Idaho | 18.17 |
| Illinois | 39.11 |
| Indiana | 41.07 |
| Iowa | 35.11 |
| Kansas | 29.96 |
| Kentucky | 49.8 |
| Louisiana | 61.01 |
| Maine | 42.04 |
| Maryland | 41.96 |
| Massachusetts | 47.75 |
| Michigan | 32.57 |
| Minnesota | 30.46 |
| Mississippi | 54.79 |
| Missouri | 42.88 |
| Montana | 15.45 |
| Nebraska | 23.65 |
| Nevada | 9.52 |
| New Hampshire | 43.58 |
| New Jersey | 46.5 |
| New Mexico | 14.57 |
| New York | 41.76 |
| North Carolina | 49.59 |
| North Dakota | 17.95 |
| Ohio | 39.07 |
| Oklahoma | 36.5 |
| Oregon | 27.55 |
| Pennsylvania | 42.66 |
| Rhode Island | 46.24 |
| South Carolina | 49.83 |
| South Dakota | 19.87 |
| Tennessee | 54.01 |
| Texas | 28.93 |
| Utah | 12.21 |
| Vermont | 37.68 |
| Virginia | 42.77 |
| Washington | 38.19 |
| West Virginia | 45.43 |
| Wisconsin | 32.48 |
| Wyoming | 14.48 |
Annual Rainfall Statistics By Region In The UK (in inches)
| Region | Annual Average (in inches) |
| Scotland | 48.82 |
| Northern Ireland | 36.07 |
| North West England | 38.12 |
| North East England | 29.63 |
| Midlands | 27.21 |
| East of England | 22.01 |
| South West England | 36.33 |
| South East England | 24.14 |
| Wales | 39.62 |
| London | 22.45 |
