What is the NIOSH lifting equation?

It is a method from the US National Institute for Occupational Safety and Health (NIOSH) for assessing two-handed manual lifting tasks. The revised equation, published in 1993 and explained in the 1994 Applications Manual (revised in 2021), works out a recommended weight limit (RWL): a load constant of 23 kg (51 lb) multiplied by six factors for how far out the hands are, how high, how far the load travels, how much the body twists, how often and how long the lifting goes on, and how good the grip is. The lifting index (LI) is the actual load divided by the RWL.

What is a good lifting index?

NIOSH's goal is to design every lifting job to an LI of 1.0 or less. It says tasks with an LI above 1.0 likely pose an increased risk of lifting-related low back pain for some fraction of the workforce, and the manual reports that experts agree nearly all workers would be at increased risk above 3.0. NIOSH gives no other bands, and it says the shape of the risk function is not known, so an LI of 1.0 or less is a design goal, not a guarantee for every person.

When can the NIOSH lifting equation not be used?

NIOSH says it does not apply to lifting or lowering with one hand, for over 8 hours, while seated or kneeling, in a restricted work space, of unstable objects, while carrying, pushing or pulling, with wheelbarrows or shovels, at high speed (faster than about 30 inches, 76 cm, a second), on a slippery floor (coefficient of friction under 0.4), or in an unfavourable environment (temperature well outside 19 to 26 °C or humidity outside 35 to 50%). Those tasks need a different assessment.

Why is my result slightly different from the NIOSH manual's examples?

The manual's worked examples round each multiplier to two decimals before multiplying, and this calculator keeps full precision, as the manual asks when it says each multiplier should be computed from its formula. The RWLs therefore differ by a few tenths of a pound; for the manual's Examples 1 to 6 this calculator lands within 3% of every printed RWL and within 0.1 of every printed LI. The manual itself says such rounding differences should not be significant.

Should I use metric or U.S. units?

Either, but use one throughout a study. NIOSH gives a metric form (23 kg, 25/H, centimetres) and a U.S. form (51 lb, 10/H, inches), and they are not exact conversions of each other: 51 lb is 23.1 kg and 10 inches is 25.4 cm. For the worked example on this page the U.S. form gives an RWL about 2% higher than the metric form.

How do I assess a job with lifts from several pallet layers or shelf heights?

This calculator does one task at a time. Either assess the worst lift (the bottom layer, say, which is what the manual does in its Example 3) with the frequency of all the lifts, or, when you need detail on every lift, use the manual's multi-task procedure, which combines the tasks into a composite lifting index (section 2.4). The calculator does not do that composite step.

Free tool

NIOSH lifting equation calculator

The revised NIOSH lifting equation gives the recommended weight limit (RWL) for one two-handed lifting task: the load that nearly all healthy workers could lift over a substantial period, up to 8 hours, without an increased risk of lifting-related low back pain. It starts from 23 kg (51 lb) and multiplies it by six factors for reach, height, travel, twist, frequency and grip. The lifting index (LI) is the actual load divided by the RWL. This calculator shows every multiplier, so you can see which part of the task to change first.

RWLequalsLC × HM × VM × DM × AM × FM × CM

LIequalsLoad weight divided by RWL

HMequals25 ÷ H

VMequals1 − 0.003 × |V − 75|

DMequals0.82 + 4.5 ÷ D

AMequals1 − 0.0032 × A

Metric form: LC = 23 kg; H, V and D in cm; A in degrees. FM and CM come from the NIOSH tables (frequency by duration and height; grip by height). U.S. form: LC = 51 lb, HM = 10 ÷ H, VM = 1 − 0.0075 × |V − 30|, DM = 0.82 + 1.8 ÷ D, in inches. Limits: H 25 to 63 cm (under 25 counts as 25, over 63 gives 0), V 0 to 175 cm, D 25 to 175 cm (under 25 counts as 25), A 0 to 135 degrees, F up to 15 lifts a minute depending on duration and height. Source: NIOSH Publication 94-110, section 1.3.

Units

Metric (kg, cm)U.S. (lb, in)

Load weight

Including the container. If it varies, use the heaviest

Start of the lift (origin)

H, hands out

From between the ankles

V, hand height

Floor to knuckles

A, twist

0 = straight ahead

End of the lift (destination)

V, hand height

Vertical travel D = 70 cm

Significant control at the destination: the operator re-grips the load, holds it there for a moment, or has to place or guide it carefully. The RWL is then worked out at both ends and the lower one counts.

Lifts per minute

Count lifts over 15 minutes and divide by 15

Lifting duration

1 hour or lessOver 1, up to 2 hOver 2, up to 8 h

Grip (coupling)

GoodFairPoor

Pre-filled with the worked example. One lifting task, done two-handed, standing, in front of the body or twisted. Duration needs recovery time after each session (see below); the equation does not cover more than 8 hours.

Lifting index (LI)

1.51

Load 12 kg ÷ recommended weight limit 7.93 kg.

Above 1.0: NIOSH says lifting tasks with an LI above 1.0 likely pose an increased risk of lifting-related low back pain for some fraction of the workforce. Redesign toward 1.0 or less.

Recommended weight limit (RWL)

7.93 kg

At the origin

Load constant

23 kg

The RWL with every multiplier at 1.0

Multipliers at the origin (1.0 means no reduction)

HM horizontal0.625

VM vertical0.865

DM distance0.884

AM asymmetric0.904

FM frequency0.84

CM coupling0.95

Biggest reduction: the horizontal multiplier (HM 0.625)

With H of 25 cm or less and nothing else changed, the RWL would be 12.69 kg (LI 0.95). Bring the load closer: remove whatever keeps the operator away from it, or make it smaller front to back.

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How to measure it

Measuring H, V, D, A and F on the floor

Bring a tape measure, a protractor or a phone angle app, a stopwatch, a scale and floor tape. Watch the real task at its normal pace and measure where the hands are, not where the box is drawn on the layout.

H, hands out. Tape a mark on the floor halfway between the operator's inner ankle bones and another straight below the middle of the hands (the large middle knuckle). H is the distance between the two marks. If you cannot measure it, NIOSH allows an estimate: H = 20 + W ÷ 2 cm when the hands are at least 25 cm off the floor, or 25 + W ÷ 2 cm below that, where W is the load's size front to back (U.S.: 8 + W ÷ 2 in, or 10 + W ÷ 2 in below 10 in).

V, hand height. Floor to the middle of the hands, at the start and at the end of the lift. The calculator works out the vertical travel D from the two.

A, twist. The angle between straight ahead (body facing forward, hands in front, no twist) and the line from the ankle mark to the mark under the hands. It is set by where the load is, not by how the feet point or how far the person twists. Always measure it at the start; at the end too when the load needs careful placing.

F, lifts per minute. Count the lifts in a typical 15 minutes and divide by 15. If the operator lifts in bursts inside those 15 minutes, use the total lifts divided by 15 (8 minutes at 10 a minute, then 7 minutes of light work, is 80 ÷ 15 = 5.33). If the rate changes by more than 2 a minute between sessions, assess each session separately.

Duration. 1 hour or less of lifting, followed by at least as much light work (recovery); over 1 and up to 2 hours, followed by recovery of at least 0.3 times the lifting time; or over 2 and up to 8 hours with normal breaks. If the recovery is shorter than that, add the sessions together: 30 minutes of lifting, 10 minutes of light work and 45 more minutes of lifting counts as 75 minutes, so over 1 hour.

Significant control. Tick it when the operator has to re-grip the load near the end, hold it there for a moment, or place or guide it carefully. Then measure H and A at the destination too; the lower RWL of the two ends is the one that counts.

Worked example

Illustrative numbers, not a benchmark

A 12 kg case of parts, a compact carton with no hand-holds that the operator grips by clamping the fingers under it, goes from a pallet layer to a bench. At the pick the hands are 40 cm out from the ankles and 30 cm off the floor, and the operator twists 30 degrees. On the bench the hands end at 100 cm. Four lifts a minute, in sessions under an hour with equal recovery time. No careful placing.

1HM = 25 ÷ 40 = 0.625. VM = 1 − 0.003 × |30 − 75| = 0.865. D = 100 − 30 = 70 cm, so DM = 0.82 + 4.5 ÷ 70 = 0.884. AM = 1 − 0.0032 × 30 = 0.904.
2FM = 0.84 (4 lifts a minute, 1 hour or less, hands under 75 cm). CM = 0.95 (fair grip, hands under 75 cm).
3RWL = 23 × 0.625 × 0.865 × 0.884 × 0.904 × 0.84 × 0.95 = 7.93 kg.
4LI = 12 ÷ 7.93 = 1.51.
5The smallest multiplier is HM. A pallet turntable that lets the operator stand right at the case brings H to 25 cm: HM = 1.0, RWL = 12.69 kg, LI = 0.95.

LI 1.51 before, 0.95 after one change. The reach cost more than the twist, the height or the rate. Raising the pick to knuckle height would also take VM and CM to 1.0 and add more margin.

Reading the lifting index

NIOSH words it like this (manual section 1.4.3), and gives no other bands:

1.0 or less is the design goal for every lifting job.
Above 1.0: likely an increased risk of lifting-related low back pain for some fraction of the workforce.
Above 3.0: the manual reports that experts agree nearly all workers would be at increased risk.

The LI is a relative measure. NIOSH says the shape of the risk function is not known, so it cannot tell you the risk for one person or the share of a crew at risk. Use it to rank tasks and to compare a task before and after a change. NIOSH says to redesign first the factor that takes the most off the load constant: the lowest multiplier.

When the equation does not apply

NIOSH lists these cases (manual section 1.2):

Lifting or lowering with one hand, or for over 8 hours
Lifting while seated or kneeling, or in a restricted work space
Unstable loads, such as some containers of liquid or part-filled bags
Lifting while carrying, pushing or pulling (carrying limited to one or two steps)
Wheelbarrows and shovels
High-speed lifts, faster than about 76 cm (30 in) a second
Slippery floors: a shoe-to-floor coefficient of friction under 0.4
Temperature well outside 19 to 26 °C, or humidity outside 35 to 50%

It also assumes lifting and lowering carry the same risk, and that other manual handling (holding, pushing, pulling, carrying, walking) is under about 10% of the work. Outside these limits use another method or a qualified ergonomist.

Grading the grip

Good: a container of optimal design with optimal handles or hand-hold cut-outs, or a loose part the hand can wrap around comfortably.
Fair: handles or cut-outs that are less than optimal, or no handles but the fingers can clamp about 90 degrees under the load, as with a carton lifted off the floor.
Poor: a container more than 40 cm front to back or 30 cm high, rough or slippery, with sharp edges, an off-centre load or unstable contents, or that needs gloves; bulky loose parts; bags that sag in the middle; or a grip where the fingers cannot reach 90 degrees.

An optimal handle is 1.9 to 3.8 cm across, at least 11.5 cm long, with 5 cm clearance, cylindrical and non-slip. If in doubt between two grades, NIOSH says take the worse one. The grip only changes the RWL when the hands are below 75 cm, except for a poor grip, which costs 10% at any height.

Sources and how this calculator was checked

NIOSH, Applications Manual for the Revised NIOSH Lifting Equation, Waters, Putz-Anderson and Garg, DHHS (NIOSH) Publication No. 94-110, revised September 2021. Equation, limits and Tables 1 to 7.
Waters, Putz-Anderson, Garg and Fine, Revised NIOSH equation for the design and evaluation of manual lifting tasks, Ergonomics 36(7), 749 to 776, 1993.

The frequency and grip tables were copied from the 2021 manual and checked cell by cell against the 1993 paper. Unit tests run the manual's worked Examples 1 to 6 and their redesigns through the calculator: every RWL lands within 3% and every LI within 0.1 of the printed values, the gap being the manual's two-decimal rounding. Between table rows the frequency multiplier is interpolated, as the manual says; above a column's last non-zero row it is 0.

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How LeanSuite helps

LeanSuite's AI Ergonomic Study scores REBA and RULA from ordinary video and flags the postures that need a fix. It does not calculate the NIOSH lifting equation; use this calculator for lifting tasks.

See Ergonomic Study

FAQ

NIOSH lifting equation calculator: common questions

It is a method from the US National Institute for Occupational Safety and Health (NIOSH) for assessing two-handed manual lifting tasks. The revised equation, published in 1993 and explained in the 1994 Applications Manual (revised in 2021), works out a recommended weight limit (RWL): a load constant of 23 kg (51 lb) multiplied by six factors for how far out the hands are, how high, how far the load travels, how much the body twists, how often and how long the lifting goes on, and how good the grip is. The lifting index (LI) is the actual load divided by the RWL.
NIOSH's goal is to design every lifting job to an LI of 1.0 or less. It says tasks with an LI above 1.0 likely pose an increased risk of lifting-related low back pain for some fraction of the workforce, and the manual reports that experts agree nearly all workers would be at increased risk above 3.0. NIOSH gives no other bands, and it says the shape of the risk function is not known, so an LI of 1.0 or less is a design goal, not a guarantee for every person.
NIOSH says it does not apply to lifting or lowering with one hand, for over 8 hours, while seated or kneeling, in a restricted work space, of unstable objects, while carrying, pushing or pulling, with wheelbarrows or shovels, at high speed (faster than about 30 inches, 76 cm, a second), on a slippery floor (coefficient of friction under 0.4), or in an unfavourable environment (temperature well outside 19 to 26 °C or humidity outside 35 to 50%). Those tasks need a different assessment.
The manual's worked examples round each multiplier to two decimals before multiplying, and this calculator keeps full precision, as the manual asks when it says each multiplier should be computed from its formula. The RWLs therefore differ by a few tenths of a pound; for the manual's Examples 1 to 6 this calculator lands within 3% of every printed RWL and within 0.1 of every printed LI. The manual itself says such rounding differences should not be significant.

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