NIOSH Lifting Equation Calculator (RWL & Lifting Index)

How it works

RWL = 23 kg × HM × VM × DM × AM × FM × CM; HM = 25/H; VM = 1 − 0.003|V − 75|; DM = 0.82 + 4.5/D; AM = 1 − 0.0032A; FM from the frequency/duration table; CM 1.0 / 0.95 / 0.90; LI = load ÷ RWL

The revised NIOSH lifting equation (1991) estimates the heaviest load that nearly all healthy workers could lift repeatedly without raising their risk of low-back injury. It starts from an ideal 23 kg and discounts it for everything that makes a lift harder: reaching out (horizontal multiplier falls steeply beyond 25 cm), picking up from the floor or above the shoulders, long vertical travel, twisting, frequent lifts over a long shift, and poor grip. Dividing the actual load by that recommended limit gives the lifting index — under 1 is acceptable, 1 to 3 warrants redesign, over 3 is a clear hazard. The multipliers also diagnose the fix: the smallest one shows which task feature to change first. It applies to two-handed, smooth lifts in unrestricted postures.

Worked example

A 15 kg box lifted from 30 cm height with hands 40 cm out, raised 60 cm with a 30° twist, twice a minute for 1–2 hours with fair grip: RWL ≈ 8.0 kg, lifting index 1.87 — increased risk; the horizontal reach (0.63) is the main penalty.

Frequently asked questions