In short
- A rainwater tank size calculator starts with roof footprint, then multiplies by Tyler's 42.1 inches of normal yearly rain and 0.623 gallons per square foot per inch.
- Real roofs lose some water, so Texas training materials use a collection factor of about 0.85. Metal roofs are generally the cleanest surface to harvest from.
- Tank size is a storage question: daily use times the dry days you want to cover. A 300 gallon-a-day household covering 60 days needs about 18,000 gallons.
- July is the pinch month: the normal is only 2.1 inches, so a roof that covers the year on paper can still run short in summer.
This rainwater tank size calculator is for Tyler and Smith County owners planning a barndominium who want to know how much water a metal roof can supply and what tank size to ask an installer to quote. Enter your roof footprint and daily water use, pick how many dry days to cover, and it shows the yearly harvest, the tank range and the driest-month gap. It pairs with our gutters and drainage guide.
En español: calculadora de tamaño de cisterna de agua de lluvia
| Your numbers |
|---|
| Month | Harvest | Use | Balance |
|---|
Example: 3,000 sq ft roof footprint, 300 gallons a day, 60 dry days, 85% collection.
How the numbers are worked out
- Gallons per inch.
1 inch on 1 sq ft is 7.48 / 12 = 0.623 gallons. For 3,000 sq ft that is 1,869 gallons per inch of rain.
- Yearly harvest.
3,000 x 42.1 inches x 0.623 = 78,685 gallons at most. At 85% collection that is 66,882 gallons.
- Range.
The range shows collection 5 points lower and 5 points higher (80% to 90% here): 62,900 to 70,800 gallons.
- Tank for dry days.
Daily use times dry days: 300 x 60 = 18,000 gallons of usable water. The top of the tank range adds a quarter for the water below the outlet and the room at the overflow, then rounds up to the next 500 gallons.
- Driest month.
July's normal is 2.1 inches: 3,000 x 2.1 x 0.623 x 0.85 = 3,336 gallons caught against 9,300 used (31 x 300), so the tank has to cover about 5,964 gallons that month.
Roof size against yearly harvest
| Roof footprint | Gallons a year | Days at 300 gal |
|---|---|---|
| 2,000 sq ft | 44,588 | 149 |
| 3,000 sq ft | 66,882 | 223 |
| 4,000 sq ft | 89,176 | 297 |
Tyler normal rain of 42.1 inches and 85% collection. A normal year is an average, not a promise: dry years bring much less. Days at 300 gal shows how long the yearly harvest would last at that use if all of it could be stored and used.
Tank size by daily use and dry days
| Daily use | 30 days | 60 days | 90 days |
|---|---|---|---|
| 150 gal | 4,500 | 9,000 | 13,500 |
| 300 gal | 9,000 | 18,000 | 27,000 |
| 450 gal | 13,500 | 27,000 | 40,500 |
Gallons of usable water, before the allowance for water you cannot draw out. Large tanks are easier to fit above ground or on a pad than to bury; ask the installer about both.
Planning numbers, not a system design. The tank supplier or installer sizes the tank, the first-flush diverter, the pump and the overflow, and a licensed plumber handles any link to house plumbing. Texas rules apply if the building also connects to a public water system. See our septic and well guide for the main water source and the gutter and downspout calculator for catching the roof flow.
Where a rain tank plan goes wrong
- Sizing the tank to the yearly total. Rain comes in bursts. A tank that holds the year's harvest would be huge; size to the dry spell.
- Counting the house roof as 100%. Gutters overshoot in hard rain and the first flush is thrown away. The first flush guideline is about 10 gallons per 1,000 sq ft of roof each time.
- Ignoring July and August. They are Tyler's driest normal months, and summer is when use climbs.
- Skipping the overflow plan. A full tank spills. Send the overflow away from the slab: see gutters and drainage.
What to ask your builder and installer
- Which roof areas drain to the tank? Ask for the footprint on the plans, shop and porch included.
- Where does the tank sit? A pad, slab or buried tank changes the site work. Check it against the slab plan.
- What is the first-flush and overflow design? Ask who supplies and who maintains them.
- Is it part of the bid? Read how to choose a barndominium builder and ask whether the builder or a separate installer does it.
Next step: get a written quote
This tool sizes a tank and estimates water, not the system or the price. We are a referral service, not a contractor; we connect visitors with independent local providers. Use the form below to ask independent barndominium builders near Tyler for a written quote that lists the roof area to the gutters, the tank size and the overflow plan, and read how to choose a barndominium builder first.
Sources
- U.S. Climate Normals 1991 to 2020, NOAA NCEI: Tyler Pounds Regional Airport rain, 42.10 inches a year; July 2.10 and June 4.38. Monthly figures as shown on our best time to build page.
- The Texas Manual on Rainwater Harvesting, Texas Water Development Board: sizing method for Texas and first-flush practice.
- Rainwater harvesting training, Texas Water Development Board: 0.62 gallons per square foot per inch and a collection factor of about 0.85; first flush of about 10 gallons per 1,000 sq ft.
- How We Use Water, EPA WaterSense: an average family uses more than 300 gallons a day at home.
Frequently asked questions
How big a rainwater tank do I need for a barndominium?
Multiply the gallons you use per day by the number of dry days you want to cover. A household using 300 gallons a day that wants 60 days of cover needs about 18,000 gallons of usable water, and the tank should be somewhat larger than that because the water below the outlet and the room at the overflow are not usable. The tank supplier or installer confirms the real sizes.
How much water does a metal roof collect in Tyler?
One inch of rain on one square foot is 0.623 gallons. Tyler's 1991 to 2020 normal is about 42.1 inches a year, and a metal roof loses a share to splash, the first flush and overflow. Texas Water Development Board training materials use a collection factor of about 0.85. A 3,000 sq ft roof footprint then yields roughly 67,000 gallons in a normal year.
Is roof area the floor area of the house?
No. Use the flat footprint the roof covers, including overhangs and any porch or shop roof that drains into the same gutters. A steep pitch does not add collection area; the rain falls on the footprint.
Can I drink rainwater collected from my roof?
It can be done, but it needs treatment such as filtration and disinfection, and if the building also connects to a public water system Texas requires safeguards against cross-connection. Ask a licensed plumber or installer, and check the rules with the county and your water provider before you plan on potable use.
Can rainwater replace a well in East Texas?
For a household, usually not on its own: Tyler's driest normal month, July, brings about 2.1 inches, which is far too little for a typical home's use. Rain tanks more often cut well or water-corporation use, or serve irrigation, livestock and a shop. Our septic and well guide covers the main supply choice.