Are there affiliate links on this page?

No. This is a free field note with a calculator. The product review in this chapter is a separate page and says so.

// chapter 03 · water · part 2 of 9 · field note · 10 September 2026

Your rain barrel is only half a system: how much water your roof really catches, by state

Q: How many gallons does a rain barrel collect per inch of rain?

About 0.62 gallons per square foot of roof per inch of rain. A 1,000 square foot roof section sends roughly 620 gallons to the gutter in a one-inch storm, far more than a 55-gallon barrel holds, which is why overflow matters more than capacity.

Q: Where does the rainfall number come from?

From NOAA's National Centers for Environmental Information, Climate at a Glance: the statewide average annual precipitation for the 1991–2020 base period, read on 10 September 2026. It is a state average; your town can be wetter or drier, so the calculator lets you type your own number.

Most people buy the barrel first and do the math never. This calculator does the math: pick your state, enter the roof section that drains to one downspout, choose a barrel, and see gallons per inch, per average month and per year, from NOAA's thirty-year rainfall averages. Then it shows the two pieces most setups forget: the first flush and the overflow.

No affiliate links on this page. It is a free tool and a field note. The product review in this chapter is a separate page and says so at the top.

// the count

How much rain does your roof send to the barrel?

One inch of rain on one square foot of roof is about 0.62 gallons (USGS puts one inch on a 40 × 70 ft roof at 1,743 gallons). The rest is your roof, your rainfall and your barrel.

Your state NOAA statewide average, 1991–2020. Pick "my own number" to type your town's rainfall.

Rain per year, in inches Fills in from your state; edit it if you know your local number.

Roof section draining to this downspout, in square feet Length × width of that roof section, as seen from above. A 40 × 25 ft section is 1,000 sq ft.

Barrel or tank size, in gallons Common sizes: 55 (barrel), 100, 275 (IBC tote), 550.

First flush you divert, in gallons per rain The dirty first runoff you send away from the barrel. Your number; 5 to 10 gal is a common setting for a small roof.

Rains per month that are big enough to fill anything Your estimate. Used only for the first-flush loss.

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per inch of rain

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in an average month

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in an average year

Average month

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Your barrel

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Overflows if unused

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Sources: USGS Water Science School, "Rain and precipitation" (1,743 gal from one inch on a 40 × 70 ft roof = 0.62 gal per sq ft; read 10 Sept 2026). NOAA NCEI, Climate at a Glance, statewide annual precipitation, base period 1991–2020 (read 10 Sept 2026). A state average hides local differences; a mountain town and a valley town in the same state can differ by a lot, so type your own number when you have it. First-flush and rain-count are your estimates, not data.

// the three pieces most people forget

The barrel is the visible part. The system is the project.

1. The first flush. The first gallons off a dry roof carry the dust, pollen, droppings and grit that settled since the last rain. A first-flush diverter is a length of pipe that fills with that dirty water first and sends the cleaner water on to the barrel. It is the part that usually keeps the barrel from smelling by July.

2. The overflow. Look at the numbers above. In most states a single 55-gallon barrel fills in the first minutes of a real storm, and everything after that has to go somewhere. If "somewhere" is your foundation, the barrel did harm. An overflow line that leads the water back to the downspout's original path, or to a second barrel, or to a garden bed that can take it, is not optional.

3. The use. A barrel that never gets emptied is a barrel that never fills again. The common rule of thumb for vegetables is about an inch of water a week in summer, which for a 4 × 8 ft bed is roughly 20 gallons. One barrel serves two or three beds for a week; the numbers above tell you how many weeks a month of rain would cover.

Rule of thumb, not data: the "inch a week" comes from general extension guidance and depends on your soil, mulch and heat. Watch the plants, not the calendar.

// watch it built

What a first flush and an overflow look like in a real setup

We have not built this system. The clearest walk-through we found of a first-flush diverter feeding IBC totes, with the overflow handled, is by the channel homesteadonomics. It is their work, shown here with their embed enabled; the credit and the link go to them.

Video by a creator we don't own

"Rainwater Harvesting System For the Horse: DIY First Flush and IBC Totes" by homesteadonomics on YouTube. A homestead channel that documents its own builds; the views and methods in the video are theirs, not ours. Watch it on YouTube to see their other water projects.

// the whole map on one page

Average rainfall by state, and what 500 sq ft of roof catches in a year

The same NOAA averages the calculator uses. Wet states fill a barrel every week; dry states need the tank and the patience.

State	Inches per year (1991–2020)	Gallons per year, 500 sq ft

Alaska and Hawaii are not in NOAA's statewide contiguous series; type your own number above. Source: NOAA NCEI Climate at a Glance, statewide time series, precipitation, January–December, base period 1991–2020, read 10 Sept 2026.

// chapter 03 · water at homenine parts, read in any order

i.How much water your house needs, and the Water Freedom System review
ii.Rain barrel calculator, by state
iii.48 hours without power: a small apartment's quiet plan
iv.A 30-day pantry, priced with this month's numbers
v.Generator vs. battery: what your state's outages say
vi.Water from air, explained by the humidity where you live
vii.This week's drought map, and your plan on top of it
viii.Filter, treat, test: making stored water drinkable
ix.The blackout kit you already own

Next: part iii, when it's published. Start with part i if you skipped it.Run the household water plan →

// straight answers

Questions people ask

How many gallons does a rain barrel collect per inch of rain?

About 0.62 gallons per square foot of roof. A 1,000 sq ft roof section sends roughly 620 gallons to the gutter in a one-inch storm, far more than a barrel holds. That is why the overflow is the design problem, not the capacity.

Where does the rainfall number come from?

NOAA's Climate at a Glance, statewide average annual precipitation for 1991–2020, read on 10 September 2026. It is a state average. Type your own number when you know your town's.

Is rain barrel water safe to drink?

Not as collected. Roof runoff carries dust, droppings and debris. Use it in the garden, or treat and test it before any drinking use. This page makes no claim about water safety; part viii of this chapter covers treatment.

Is collecting rain legal where I live?

In most of the U.S., yes, and a few states limit how much you may store or require a permit for larger systems. We don't keep a legal table here because rules change; check your state's water agency before buying a big tank. Our rainwater basics guide lists the questions to ask.

Last updated: 10 September 2026. Numbers are averages from public sources with the dates shown; your roof and your weather will differ.

How we stay honest: this page has no affiliate links and no sponsor. We have not built the system shown in the video; it belongs to the creator credited above. The calculator is arithmetic on public data with dates shown. Nothing here is a claim about the quality or safety of any water supply, and nothing here is professional advice.