Start with the formula, because it usually changes people’s plans:
Gallons = roof area (sq ft) × rainfall (inches) × 0.623 × runoff coefficient
Use 0.95 for a metal roof and about 0.85 for asphalt shingle. A 1,500 sq ft roof in one inch of rain yields roughly 1,500 × 1 × 0.623 × 0.85 = 794 gallons. That number surprises most people twice — first because a single ordinary rainfall fills a 50-gallon barrel many times over, and second because it means a barrel is not a water supply, it is a bucket that overflows.
Deciding what you are actually trying to do is therefore the first step, not choosing a tank.
What are you collecting for?
Garden irrigation only. The simplest possible setup: a barrel or two on a downpipe, a screen over the inlet, a tap at the bottom, and an overflow directed away from the foundation. No pump if you can water by gravity, no filtration, no first-flush diverter strictly necessary though it helps. This is a weekend job and costs very little.
Meaningful outdoor supply through a dry spell. Now you need real storage — hundreds or thousands of gallons — a proper tank, a first-flush diverter, and usually a pump. This is the sizing exercise below.
Indoor non-potable use (toilet flushing, laundry). Add filtration, a pump, pressure control, backflow prevention, and almost certainly a permit and inspection.
Drinking water. Add disinfection and treatment, and take local regulation seriously. This is not a DIY guide for that case.
Sizing the tank
Tank size is a balance between what your roof supplies, what you use, and how long your dry periods run. The practical method:
- Find your monthly rainfall from local climate data.
- Calculate monthly yield with the formula above.
- Estimate demand. A useful rule for garden use is around 0.6 gallons per square foot of bed per week in warm weather.
- Size for the dry gap, not the average. The tank needs to carry you through your longest typical dry stretch. In a climate with even year-round rain, a small tank refills constantly and large storage is wasted money. In a climate with a three-month dry season, storage is the entire system.
A common outcome: people in even-rainfall climates over-buy tank capacity, and people in Mediterranean or monsoon climates under-buy it badly.
| Roof area | 1 inch of rain (asphalt) | 1 inch (metal) |
|---|---|---|
| 500 sq ft | 265 gallons | 296 gallons |
| 1,000 sq ft | 530 gallons | 592 gallons |
| 1,500 sq ft | 794 gallons | 888 gallons |
| 2,000 sq ft | 1,059 gallons | 1,184 gallons |
The components, in order
Catchment (roof). Metal sheds water best and leaches least. Asphalt shingle is fine for garden use. Avoid collecting from roofs with treated wood shakes, lead flashing, or moss treatments if the water touches anything edible.
Gutters and leaf screening. Mesh over the gutter or a leaf eater at the head of the downpipe. Leaves in the tank are the main cause of foul water.
First-flush diverter. The first water off a roof carries the dust, bird droppings and debris that accumulated since the last rain. A diverter captures and discards that initial volume before the rest reaches the tank. Standard sizing is about 1 gallon of diversion per 100 sq ft of roof, more in dusty or polluted areas. Built from PVC, a 4-inch pipe holds roughly a gallon per 18 inches of length, so a 1,500 sq ft roof needs about 15 gallons — around 22 feet of 4-inch pipe, usually split across several downpipes.
Tank. Opaque, to stop algae. Sealed and screened at every opening, to stop mosquitoes breeding — this is a genuine public health issue and in many places a legal requirement. Sited on a firm level base: water weighs about 8.3 lb per gallon, so a 1,000-gallon tank is over four tons when full.
Overflow. Sized at least as large as the inlet and directed well away from the building. An overflowing tank against a foundation causes more expensive damage than the system saves.
Pump, if needed. Gravity works if the tank is uphill of the beds. Otherwise a small pump. Every foot of tank height gives about 0.43 psi, so elevating a tank even a few feet helps a gravity system considerably.
Is rainwater harvesting legal?
Almost everywhere, yes, and many places actively encourage it with rebates. But rules vary more than people expect: some western US states historically restricted collection under prior-appropriation water law, some jurisdictions cap barrel volume without a permit, and any indoor plumbing connection triggers building code requirements including backflow prevention.
Check with your local water authority and building department before buying tanks. It is a phone call, and it occasionally saves an expensive mistake.
Do you actually need this?
Rainwater harvesting is satisfying and genuinely useful in dry-summer climates and for gardeners who irrigate heavily. But in a household context, it is rarely the largest available water saving. A leaking toilet can waste 1,000 to 4,000 gallons a day — more than a large tank holds — which is why finding and fixing that is the first move for anyone serious about water use. Harvesting makes most sense once the leaks are gone and the demand is outdoors.