Compute the Recommended Weight Limit (RWL) and Lifting Index (LI) from the revised 1991 NIOSH lifting equation. Enter the lift geometry, frequency, duration, and coupling to evaluate manual-lifting risk. Everything recomputes live in your browser.
The revised NIOSH lifting equation (1991) is the most widely used ergonomic tool for assessing the risk of two-handed manual lifting tasks. It computes a Recommended Weight Limit (RWL) — the load that nearly all healthy workers could lift over a shift without elevated risk of low-back injury — and a Lifting Index (LI) that compares the actual load to that limit. This calculator evaluates all six task multipliers plus the load constant, and color-codes the resulting Lifting Index. Calculations run live in your browser.
RWL = LC × HM × VM × DM × AM × FM × CM. The Load Constant (LC) is 51 lb — the maximum recommended load under ideal conditions. Each multiplier is a fraction between 0 and 1 that reduces the limit as the task departs from ideal: HM = 25/H (horizontal distance of the hands from the ankles, in inches); VM = 1 − 0.0075·|V − 30| (height of the hands at the start of the lift); DM = 0.82 + 1.8/D (vertical travel distance); AM = 1 − 0.0032·A (trunk twist angle in degrees); FM (frequency multiplier from a lookup table); and CM (coupling/grip multiplier). Multiply them together and only well-designed lifts keep the RWL close to 51 lb.
The frequency multiplier (FM) depends on three things: how many lifts occur per minute, how long the lifting continues (≤1, ≤2, or ≤8 hours), and whether the lift starts below or above 30 inches. Faster, longer-duration lifting drives FM down sharply, and certain high-frequency/long-duration combinations make lifting unacceptable (FM = 0). The coupling multiplier (CM) rewards good hand-to-object coupling: handles or well-shaped grips earn CM = 1.00 (Good), adequate grips earn Fair, and poor grips (e.g., loose bags, no handles) earn CM = 0.90 (Poor), with the Fair value depending on lift height. This tool uses a simplified version of the official NIOSH tables.
LI = Load Weight ÷ RWL. An LI of 1.0 or less means the task is within the recommended limit for nearly all workers. As LI rises above 1.0, the risk of low-back disorder increases and a growing fraction of the workforce is at risk. NIOSH and most ergonomists treat LI ≤ 1.0 as the design goal, 1.0 < LI ≤ 3.0 as a task that should be redesigned, and LI > 3.0 as a high-risk task that many workers cannot perform safely. To lower the LI, reduce the load, bring it closer to the body (increase HM), start the lift at knuckle height (improve VM), reduce travel and twist, slow the pace, or add handles.
The equation applies to two-handed, smooth lifting in the sagittal/asymmetric plane under reasonable conditions — moderate temperature and humidity, good footing, and unrestricted posture. It does NOT cover one-handed lifting, carrying more than a step or two, pushing or pulling, lifting while seated or kneeling, high-speed or jerking lifts, or lifting of unstable loads. For tasks that change throughout the day, NIOSH provides a Composite Lifting Index (CLI) that combines multiple lifting tasks. Always treat the RWL as a screening guideline, not a guarantee — professional ergonomic judgment is required for borderline or complex jobs.
The RWL is the maximum load that nearly all healthy workers (about 90% of adults, including most women) could lift repeatedly over a workday without an elevated risk of low-back injury, given the specific geometry and frequency of the task. It starts from a load constant of 51 lb and is reduced by six multipliers that account for how far from ideal the lift is.
An LI above 1.0 means the actual load exceeds the recommended limit, so the task poses an increased risk of low-back disorder for some portion of the workforce. The higher the LI, the greater the risk and the larger the fraction of workers who cannot perform the lift safely. Tasks above 1.0 should be redesigned; tasks above 3.0 are considered high risk.
The horizontal multiplier is HM = 25/H. Below about 10 inches the hands are essentially against the body and HM is capped at 1.0; the equation is not validated for distances closer than that because the load is effectively held at the body. This calculator requires H ≥ 10 in and caps HM at 1.0.
Reduce the weight, move the load closer to the body, start and end the lift near knuckle height, shorten the vertical travel, eliminate trunk twist, reduce the lifting frequency or duration, and improve the grip with handles or better packaging. Each change improves one of the multipliers and raises the RWL, which lowers the LI.
No. It implements the revised NIOSH equation with a simplified frequency/coupling lookup and is excellent for screening, training, and study. But real workplaces have variability, multi-task days, and conditions outside the equation’s scope. For compliance or injury investigations, have a qualified ergonomist measure the task and apply the full NIOSH method, including the Composite Lifting Index where appropriate.