The Homestead Calculator

When we first started seriously looking at rainwater collection, we made the mistake most people make: we went straight to shopping. We started pricing tanks, reading reviews of barrel pumps, watching videos about gutter filters. We spent hours on it before realizing we were building toward a number we didn't actually know yet.

The number is this: how many gallons do you actually need, and how many can your roof realistically provide? Everything else — tank size, number of downspouts, filter type — flows from that. And those two numbers are simpler to get than you'd think.

Start With What You're Collecting From

Your roof is your collection surface. The bigger it is and the more rain it receives, the more water you can harvest. The formula that ties these together is straightforward:

The 0.623 is a constant that converts inches of rainfall on square feet into US gallons. Your efficiency factor accounts for what you lose to evaporation, splash, and the first-flush diversion (more on that below). For an asphalt shingle roof, figure about 90% efficiency. Metal roofs run closer to 95%. Flat or gravel roofs drop to 60% or lower.

So a 1,500 square foot asphalt shingle roof in an area that gets 45 inches of rain annually can collect:

1,500 × 45 × 0.623 × 0.90 = approximately 37,900 gallons per year.

That's over 3,100 gallons per month on average — though the monthly distribution will vary significantly depending on your region's rainfall pattern. Tennessee, for example, gets rain fairly evenly through the year with a slight spring peak, which is actually ideal for collection planning.

Know Your Rainfall Before You Plan

Annual rainfall averages don't tell the whole story. What matters for tank sizing is your dry season — the stretch of weeks or months when you'll be drawing down your stored water faster than rain is refilling it.

In most of Tennessee, summer is the softer rainfall period. You might collect heavily from March through May and then slow down through July and August while your garden is at peak demand. That gap is what your tank needs to bridge.

Look up your location's monthly rainfall averages at weather.gov or through your state's climatology office. You're looking for:

• Your driest consecutive two or three months
• How much you'd collect in those months versus how much you'd use
• The deficit — that's your minimum tank size

Figure Out What You Actually Need

This is where people are usually too vague. "Enough for the garden" isn't a number. You need a number.

A vegetable garden needs roughly one inch of water per week during the growing season — about 26 inches total over a typical season. One inch of water on one square foot equals 0.623 gallons, so a 400 square foot garden needs about 250 gallons per week at peak, or roughly 6,500 gallons over a full season.

For livestock, daily water needs are:

Animal	Gallons / Day	Annual Total
Chickens (per 10 birds)	1–2 gal	365–730 gal
Goat (per animal)	1–3 gal	365–1,095 gal
Pig (per animal)	3–5 gal	1,095–1,825 gal
Beef cow (per animal)	20–30 gal	7,300–10,950 gal

Add in any household non-potable uses — toilet flushing, laundry, washing down barn floors — and you start to have a real daily number to plan around.

Sizing Your Tank

The single most common rainwater system mistake is undersizing the tank. A 55-gallon rain barrel looks great on a porch, and for topping up a container garden it works fine. For anything serious — irrigating a half-acre garden, watering a small flock, or bridging a six-week dry stretch — you need to think in thousands of gallons, not dozens.

The rule of thumb we use: size your tank to hold at least 1.5 times your peak monthly collection, or your full dry-season deficit, whichever is larger.

For most homesteads planning serious production:

• 500–1,000 gallons: Small kitchen garden, minimal livestock. Fine as a starting point.
• 1,500–3,000 gallons: Productive garden up to 800 sq ft, small flock, one or two goats.
• 5,000–10,000 gallons: Serious market or self-sufficiency garden, mixed livestock, partial household supply.
• 10,000+ gallons (cistern): Full household non-potable supply, large livestock operation, or multi-month drought buffer.

IBC totes (275 or 330 gallon food-grade intermediate bulk containers) are the most cost-effective option in the 1,000–3,000 gallon range. They run $100–$300 used, connect easily, and can be stacked or linked. For larger volumes, buried polyethylene tanks or concrete cisterns are more practical.

The First-Flush Diverter — Don't Skip This

The first rain after a dry spell washes everything off your roof — bird droppings, dust, pollen, decomposed leaves, atmospheric pollutants. You don't want that in your storage tank. A first-flush diverter automatically routes the first few gallons of runoff away from your tank and lets the cleaner water that follows go into storage.

For a 1,000 square foot roof section, a first-flush diverter that holds about 10–15 gallons handles most roof contamination. They're inexpensive, simple to install, and dramatically improve water quality — especially if the water is going anywhere near animals or food plants.

A Note on Legality

Rainwater harvesting is legal in Tennessee with no restrictions — the state actively encourages it. Most US states are similarly permissive. The handful of states that historically had restrictions (looking at you, Colorado, though even that has since relaxed) are the exception. If you're not in Tennessee, check your state's regulations before building anything large, but for most of the country you're clear.

The Right Order of Operations

If we were starting this from scratch — which, honestly, we basically are — here's the sequence we'd follow:

1. Find out your roof's square footage (measure or pull from your home's records)
2. Look up your area's monthly rainfall averages
3. Estimate your actual water needs by use (garden, livestock, household)
4. Run the numbers to find your annual potential and monthly breakdown
5. Identify your dry-season deficit — that drives your tank size
6. Choose your collection surface (which roof sections drain where)
7. Plan your filtration: first-flush diverter at minimum, sediment filter if for livestock
8. Then, and only then, start shopping for hardware

It's not a glamorous sequence. But it's the one that keeps you from buying a 500-gallon tank when you needed a 3,000-gallon one, or building an elaborate system for a roof that can only collect a fraction of what you need.

The rainwater calculator on this site was built to handle steps 1 through 5. Put in your numbers and it'll show you monthly collection, annual totals, dry season gaps, and recommended tank sizing. Start there before you start shopping.