Spring IPPT = door weight × drum radius ÷ turns. A 182 lb, 7 ft door on 4 inch cable drums needs 47.4 IPPT (inch-pounds per turn) in total, 23.7 for each of two springs, which a .218 inch wire, 2 inch inside diameter spring 24 3/4 inches long supplies. Torsion springs are dangerous to wind: use this to identify and order the right spring, and have a trained technician install it.
How the garage door spring size is calculated
A torsion spring balances the door by storing turns of torque. Three steps get from the door to the spring.
turns = door height ÷ drum circumference + 1
total IPPT = door weight × drum radius ÷ turns; each spring = total ÷ number of springs
IPPT of a spring = 28,500,000 × wire⁵ ÷ (10.8 × (inside diameter + wire) × length)
A standard residential drum is 4 inches across, so 12.57 inches of cable unwinds per turn and a 7 ft (84 in) door needs 84 ÷ 12.57 + 1 = 7.68 turns; an 8 ft door needs 8.64. The last line is the standard torsion spring rate, IPPT = E × d⁴ ÷ (10.8 × mean diameter × active coils), with E = 28,500,000 psi for spring wire and active coils equal to length divided by wire size. A spring within 90 to 110% of the door's requirement is correctly sized.
Worked example: 16 × 7 ft insulated steel door
| Step | Calculation | Result |
|---|---|---|
| Door weight | 16 × 7 ft, steel insulated 1 3/8 in | 182 lb |
| Turns | 84 ÷ 12.57 + 1 | 7.68 |
| Total IPPT | 182 × 2 in ÷ 7.68 | 47.4 |
| Each of two springs | 47.4 ÷ 2 | 23.7 |
| Spring | .218 wire, 2 in ID, 24 3/4 in long | 23.7 IPPT |
Other wire and inside diameter pairs reach the same IPPT: .207 wire at 1.75 in ID needs 21 3/4 in, and .225 wire at 2 in ID needs 29 in.
Garage door spring size chart by door weight
For a 7 ft door on 4 inch drums with two springs, the first choice with a 2 inch inside diameter:
| Door weight | Total IPPT | IPPT per spring | Spring |
|---|---|---|---|
| 100 lb | 26.0 | 13.0 | .192 wire, 2 in ID, 24 1/4 in |
| 150 lb | 39.0 | 19.5 | .207 wire, 2 in ID, 23 1/4 in |
| 182 lb | 47.4 | 23.7 | .218 wire, 2 in ID, 24 3/4 in |
| 220 lb | 57.3 | 28.6 | .225 wire, 2 in ID, 24 in |
| 300 lb | 78.1 | 39.0 | .243 wire, 2 in ID, 25 1/2 in |
An 8 ft, 185 lb door needs 8.64 turns and 42.8 IPPT in total, 21.4 each: .218 wire, 2 in ID, 27 1/4 in.
IPPT by wire size and length (2 inch inside diameter)
| Wire size | 22 in long | 26 in long | 30 in long |
|---|---|---|---|
| .207 | 20.7 | 17.5 | 15.1 |
| .225 | 31.1 | 26.3 | 22.8 |
| .243 | 45.3 | 38.3 | 33.2 |
| .250 | 52.1 | 44.1 | 38.2 |
| .262 | 65.5 | 55.4 | 48.0 |
A longer spring is a softer spring, a thicker wire a much stronger one, since the wire size counts to the fifth power. These figures come from the formula, and published charts round differently: Garage Door Answers lists 36.2 IPPT for .225 wire, 1.75 in ID, 25 in long, where the formula gives 30.8. Treat any chart as a guide and match a replacement to the measured spring and the door's real weight.
How much does a garage door weigh?
The estimate comes from GarageDoorGuide's chart (25-gauge steel, one strut): 16 × 7 ft doors weigh 155 lb uninsulated, 170 lb vinyl-back insulated, 182 lb with 1 3/8 in insulated steel and 220 lb with 2 in insulated steel; 9 × 7 ft doors weigh 85, 95, 105 and 129 lb. Wood doors run about 300 lb single and 500 lb double, and full-view glass doors 300 to 400 lb single and over 500 lb double. Sizes off the chart use the chart's average lb per square foot. To weigh your door, disconnect the opener, lift one side onto a scale and double the reading, as Metro Garage Doors suggests. A wrong weight is the most common reason a new spring feels off.
Torsion spring safety
Winding a spring stores the door's full lifting force at head height; a slipped winding bar injures people every year. The calculator stops at identifying the spring. Springs also come in left-wound and right-wound pairs, and wire size is measured over 10 to 20 coils, not with a caliper, as IDC Spring explains. Ask the supplier to check the numbers against your old springs.
The next questions
- Replacing the door itself? See the garage door size chart for the opening and headroom, and the garage door replacement cost.
- Measuring the opening in feet and inches? Use the feet and inches calculator.
- Building the garage entry? The stair calculator and the concrete slab calculator cover the step and floor.
- See every heating, cooling and plumbing calculator.