Load index: what the number after the size actually promises
121 does not mean 121 of anything. It is the eighty-something-th rung of an internationally agreed ladder, and the rung above it is worth about three percent.
Every tire carries a service description immediately after its size — 121/118S, 104T,
95H — and the leading number is the load index. It is not a mass, a percentage or a rating scale
anyone invented for marketing. It is a position in a published series that maps each index to a
maximum load in kilograms, identically for every manufacturer in every market.
The full series is rendered below this text, computed from that standard rather than retyped, with the pound column derived by conversion so the two columns cannot drift apart.
The series is not linear, and the spacing tells you something
Adjacent indices differ by roughly 3%. Index 100 is 800 kg; 101 is 825. Up at the truck end, 116 is 1250 kg and 117 is 1285. The absolute steps grow because the proportional step stays put.
Two practical consequences follow. Ten indices is worth about a third more load, so the difference between a 100 and a 110 is substantial rather than marginal. And a single index of difference — the sort of gap that separates two otherwise identical tires on a shelf — is worth around 3%, which is real but rarely decisive on its own.
What the index is measured at
The figure is the maximum load the tire may carry at the inflation pressure that tire is rated to, on a single wheel position, at or below the speed its speed symbol permits. Every one of those conditions is load-bearing:
- Drop the pressure and the permitted load drops with it, along the curve in the manufacturer's load and inflation table. Pressure and load tables covers how that curve is read.
- Exceed the speed symbol and the load index no longer applies at all.
- Fit the tire in a dual position and the applicable figure is the second, lower number.
Two numbers with a slash
121/118 is not a range. The first index applies when the tire is fitted singly, the second when
it is fitted as one of a pair on a dual-rear-wheel axle. The dual figure is lower — inner and outer
tires do not share load perfectly, they run hotter side by side, and the standard accounts for
that. A single-wheel truck never uses the second number; a dually never uses the first.
Why four tires do not add up to your vehicle
Multiply an index by four and you get a number that is almost always larger than the vehicle's gross weight rating, sometimes by a wide margin. That is not slack you can spend.
- Load is not distributed evenly. A pickup with a loaded bed puts far more than a quarter of the total on each rear tire, and the rear axle's own rating is the binding limit.
- The vehicle manufacturer chose the original size against the axle ratings, the inflation on the door placard, and its own margins — not against the tire's ceiling.
- A P-metric passenger tire used on a light truck, van or trailer has its load capacity reduced by the standards body's divisor before it is compared with anything. The same casing carries less on a truck than it does on a car, by rule.
The placard on the driver's door jamb is where your vehicle's own numbers live, and it states the pressures those numbers assume.
Reading it off a real sidewall
The index sits between the size and the speed symbol, with no space in most markings: in
LT285/70R17 121/118S, the size is LT285/70R17, the indices are 121 and 118, and S is the
speed symbol. Alongside them, in smaller text, the sidewall repeats the same information in words —
"max load 1450 kg at 550 kPa (80 psi) cold" — and that pressure is the one the index assumes, not
the one your vehicle's placard asks for.
Where a tire is marked XL or Reinforced, its higher index comes from a higher reference inflation rather than a different casing size; XL and Reinforced markings explains the mechanism. Light-truck tires use letter load ranges instead, which load range and ply rating decodes.
Where the series runs out
Below about 70 the indices belong to small spare tires, trailers and equipment; above 130 they run into commercial and truck-bus territory, where the printed marking is usually accompanied by a full load-and-inflation table moulded into the sidewall. The rows below cover the range light vehicles actually use.
The load and speed index decoder will take a whole service description and return both halves at once.
Load index 70 to 140
| Index | Per tire | Kilograms | Four tires |
|---|---|---|---|
| 70 | 739 lb | 335 kg | 2,954 lb |
| 71 | 761 lb | 345 kg | 3,042 lb |
| 72 | 783 lb | 355 kg | 3,131 lb |
| 73 | 805 lb | 365 kg | 3,219 lb |
| 74 | 827 lb | 375 kg | 3,307 lb |
| 75 | 853 lb | 387 kg | 3,413 lb |
| 76 | 882 lb | 400 kg | 3,527 lb |
| 77 | 908 lb | 412 kg | 3,633 lb |
| 78 | 937 lb | 425 kg | 3,748 lb |
| 79 | 963 lb | 437 kg | 3,854 lb |
| 80 | 992 lb | 450 kg | 3,968 lb |
| 81 | 1,019 lb | 462 kg | 4,074 lb |
| 82 | 1,047 lb | 475 kg | 4,189 lb |
| 83 | 1,074 lb | 487 kg | 4,295 lb |
| 84 | 1,102 lb | 500 kg | 4,409 lb |
| 85 | 1,135 lb | 515 kg | 4,542 lb |
| 86 | 1,168 lb | 530 kg | 4,674 lb |
| 87 | 1,202 lb | 545 kg | 4,806 lb |
| 88 | 1,235 lb | 560 kg | 4,938 lb |
| 89 | 1,279 lb | 580 kg | 5,115 lb |
| 90 | 1,323 lb | 600 kg | 5,291 lb |
| 91 | 1,356 lb | 615 kg | 5,423 lb |
| 92 | 1,389 lb | 630 kg | 5,556 lb |
| 93 | 1,433 lb | 650 kg | 5,732 lb |
| 94 | 1,477 lb | 670 kg | 5,908 lb |
| 95 | 1,521 lb | 690 kg | 6,085 lb |
| 96 | 1,565 lb | 710 kg | 6,261 lb |
| 97 | 1,609 lb | 730 kg | 6,437 lb |
| 98 | 1,653 lb | 750 kg | 6,614 lb |
| 99 | 1,709 lb | 775 kg | 6,834 lb |
| 100 | 1,764 lb | 800 kg | 7,055 lb |
| 101 | 1,819 lb | 825 kg | 7,275 lb |
| 102 | 1,874 lb | 850 kg | 7,496 lb |
| 103 | 1,929 lb | 875 kg | 7,716 lb |
| 104 | 1,984 lb | 900 kg | 7,937 lb |
| 105 | 2,039 lb | 925 kg | 8,157 lb |
| 106 | 2,094 lb | 950 kg | 8,378 lb |
| 107 | 2,150 lb | 975 kg | 8,598 lb |
| 108 | 2,205 lb | 1,000 kg | 8,818 lb |
| 109 | 2,271 lb | 1,030 kg | 9,083 lb |
| 110 | 2,337 lb | 1,060 kg | 9,348 lb |
| 111 | 2,403 lb | 1,090 kg | 9,612 lb |
| 112 | 2,469 lb | 1,120 kg | 9,877 lb |
| 113 | 2,535 lb | 1,150 kg | 10,141 lb |
| 114 | 2,601 lb | 1,180 kg | 10,406 lb |
| 115 | 2,679 lb | 1,215 kg | 10,714 lb |
| 116 | 2,756 lb | 1,250 kg | 11,023 lb |
| 117 | 2,833 lb | 1,285 kg | 11,332 lb |
| 118 | 2,910 lb | 1,320 kg | 11,640 lb |
| 119 | 2,998 lb | 1,360 kg | 11,993 lb |
| 120 | 3,086 lb | 1,400 kg | 12,346 lb |
| 121 | 3,197 lb | 1,450 kg | 12,787 lb |
| 122 | 3,307 lb | 1,500 kg | 13,228 lb |
| 123 | 3,417 lb | 1,550 kg | 13,669 lb |
| 124 | 3,527 lb | 1,600 kg | 14,110 lb |
| 125 | 3,638 lb | 1,650 kg | 14,551 lb |
| 126 | 3,748 lb | 1,700 kg | 14,991 lb |
| 127 | 3,858 lb | 1,750 kg | 15,432 lb |
| 128 | 3,968 lb | 1,800 kg | 15,873 lb |
| 129 | 4,079 lb | 1,850 kg | 16,314 lb |
| 130 | 4,189 lb | 1,900 kg | 16,755 lb |
| 131 | 4,299 lb | 1,950 kg | 17,196 lb |
| 132 | 4,409 lb | 2,000 kg | 17,637 lb |
| 133 | 4,542 lb | 2,060 kg | 18,166 lb |
| 134 | 4,674 lb | 2,120 kg | 18,695 lb |
| 135 | 4,806 lb | 2,180 kg | 19,224 lb |
| 136 | 4,938 lb | 2,240 kg | 19,753 lb |
| 137 | 5,071 lb | 2,300 kg | 20,283 lb |
| 138 | 5,203 lb | 2,360 kg | 20,812 lb |
| 139 | 5,357 lb | 2,430 kg | 21,429 lb |
| 140 | 5,512 lb | 2,500 kg | 22,046 lb |
71 indexes shown. Kilogram figures are the standard published series; pound figures are converted from them here, so the two columns cannot disagree.