Elevator hoist rope safety factor calculator

Factor of safety of elevator suspension ropes from rope count, roping, breaking load, car mass, rated load and the rope hanging in the hoistway, checked against EN 81-20 or the ASME A17.1 minimum for your speed.

CodeThe elevator code the job is built to. Under ASME, enter the minimum from the code's table for your speed.

DriveEN 81-20 sets a different minimum for each.

Number of ropes

Separate suspension ropes. A whole number.

Roping ratio

1 for 1:1, 2 for 2:1 and so on: how many runs of each rope hold the car.

Minimum breaking load of one rope

From the rope maker's certificate. (lbf)

Empty car mass

Car, sling, platform and everything that hangs on the car ropes, with no load. (lb)

Rated load

The load the car is rated to carry. (lb)

Travel

Bottom landing to top landing: sets how much rope hangs below the sheave. (ft)

Further minimum factor (0 for none)

Under ASME, the figure from the code's table for your rope speed and car type. Under EN, enter the verified EN 81-50 figure for your sheaves, even if it is below the fixed minimum. With 0 the EN check is incomplete.

Allowances (1)

Rope mass per metre

Mass of one metre of one rope, from the certificate (estimate shown). (lb/ft)

Link to these inputs Opening or sharing this link puts your inputs in a URL sent to the server, which may remain in browser history and request logs.

Result

Factor of safety: 12.81
Minimum checked: 12
Meets the minimum: incomplete check
Largest static load on the car ropes: 6,315 lbf
Tension in each rope run: 1,053 lbf
Rope hanging on the car side: 141 lb
Rope runs: 6

Not engineering advice; a licensed professional checks every result. Not a substitute for a licensed engineer, the manufacturer's data or the authority having jurisdiction; confirm the code edition your jurisdiction has adopted. First build: the code limits used here await a trade check against a licensed copy of the cited edition. Terms of use and estimate disclaimer.

Drawing

Factor of safety on a dial, with the minimum that applies marked.

Working

Rope runs carrying the car (ropes times the roping ratio)
$N = n \, r$
N = 6 × 1 = 6
= 6 runs
Rope hanging on the car side with the car at the bottom landing
$m_r = N \, m' \, H$
m_r = 6 × 0.35 × 30.48 ≈ 64.008 kg
= 64 kg
Largest static load on all the car ropes
$W = (P + Q + m_r)\,g$
W = (1200 + 1600 + 64.008) × 9.80665 ÷ 1000 ≈ 28.0863 kN
= 28.09 kN (rounded up)
Factor of safety
$f = \dfrac{S \, N}{W}$
f = 60 × 6 ÷ 28.0863240532 ≈ 12.8176
= 12.81 (rounded down)
Fixed minimum checked so far (EN check incomplete)
$f_{\min}$
f_min = max(12, 0) = 12
= 12

Working is shown in the units each formula is written in; the result figures above follow the unit switch. A numeric input is taken to 15 significant digits before it is used (1033.2293579541322 kg is used as 1033.22935795413 kg); every echo of it shows that value, and a computed figure substituted into the working is quoted to 15 significant digits as well.

Code checks

PASS
At least 2 independent ropes
EN 81-20:2020, 5.5.1. 6 ropes
PASS
Fixed minimum only: factor of safety at least 12 for a traction drive with three or more ropes
EN 81-20:2020, 5.5.2.2. 12.81 worked out

Notes

EN 81-20 also requires the factor to be at least the figure EN 81-50 5.12 gives for the sheaves and pulleys the rope passes over (its bends and its sheave to rope diameter ratio). Incomplete check: enter that verified figure as the further minimum, even if it is below the fixed minimum. This calculator does not work it out yet and cannot establish an overall EN pass without it.
The rope figures come from the rope maker's certificate: the minimum breaking load of one rope and its mass per metre. The default mass per metre is an estimate.
Left out: the travelling cable, compensating ropes or chains, and sheave and pulley masses that hang on the car ropes. Add them to the car mass if they apply to your layout.
Converted US input values are rounded for display only. The calculation keeps the SI values until you edit a field.

Method

What the factor of safety measures

The ropes that hold an elevator car are sized with a large margin over the load they carry. The factor of safety is that margin: the breaking strength of all the rope runs together divided by the heaviest static load they hold.

The heaviest static load comes with the rated load in the car at the bottom landing, where the most rope hangs below the drive sheave. It is the car, the load and that hanging rope, times gravity. With 2:1 roping each rope passes down to the car and back up, so twice as many runs share the load.

Minimums under each code

EN 81-20 asks for at least two ropes and a factor of at least 12 for a traction drive with three or more ropes, 16 for a traction drive with two, and 12 for a drum drive. It also asks for at least the figure EN 81-50 gives for the sheaves and pulleys the rope bends over; enter that verified figure even if it is below the fixed minimum. With none entered, a failed fixed-minimum or rope-count check still gives no; otherwise the overall check is incomplete.

ASME A17.1 writes the factor as f = S N / W, with S the rated breaking strength of one rope, N the number of runs and W the largest static load on the car ropes. Its minimum rises with the rope speed and differs for passenger and freight elevators, and it is set by a table this site does not reproduce, so you enter the figure for your speed.

Formulas

Rope runs

N = n r

N = n \, r

Largest static load

W = (P + Q + N m' H) g

W = (P + Q + N m' H)\,g

Factor of safety

f = S N / W

f = \dfrac{S \, N}{W}

Code basis: clause and edition

Standard	Edition	Clause	What it covers
ASME A17.1/CSA B44	2019	2.20.3	Factor of safety of suspension ropes, f = S N / W; minimum read from the code's table for the rope speed
EN 81-20	2020	5.5.1 and 5.5.2.2	Number of suspension ropes and their minimum safety factor by drive and rope count

Rules are restated in our own words and computed for your inputs; no table or text from a standard is reproduced. The adopted edition varies by jurisdiction, so confirm the one your authority having jurisdiction enforces.

Inputs

Input	Unit	Range	Default
Code `standard`	choice	en, asme	en
Drive `drive`	choice	traction, drum	traction
Number of ropes `ropes`	count	1 to 20	6
Roping ratio `roping`	count	1 to 4	1
Minimum breaking load of one rope `breakingLoad`	kilonewtons (kN)	5 to 2000	60
Empty car mass `carMass`	kilograms (kg)	100 to 20000	1200
Rated load `ratedLoad`	kilograms (kg)	50 to 13500	1600
Travel `travel`	metres (m)	0 to 600	30.48
Rope mass per metre `ropeMass`	kilograms per metre (kg/m)	0 to 10	0.35
Further minimum factor (0 for none) `minimumFactor`		0 to 40	0

Worked examples

Each example below is a test the calculator must pass before it ships. The expected values were worked out by a separate implementation.

Example	Expected result (each in its declared unit)
Six 60 kN ropes at 1:1, 1200 kg car, 1600 kg load, 30 m travel, EN traction	factorOfSafety: 12.82, requiredFactor: 12, staticLoad: 28.08 kN, tensionPerRun: 4.68 kN, ropeMassHanging: 63 kg, ropeRuns: 6, passes: incomplete check
Two ropes at 2:1 under EN: checked against the two-rope minimum of 16	factorOfSafety: 22.94, requiredFactor: 16, staticLoad: 10.47 kN, tensionPerRun: 2.616 kN, ropeMassHanging: 16.8 kg, ropeRuns: 4, passes: incomplete check
ASME, five 45 kN ropes at 1:1, with a minimum of 10 entered	factorOfSafety: 7.52, requiredFactor: 10, staticLoad: 29.9 kN, tensionPerRun: 5.979 kN, ropeMassHanging: 60 kg, ropeRuns: 5, passes: no
ASME with no minimum entered: the factor only	factorOfSafety: 17.73, requiredFactor: 0, staticLoad: 18.05 kN, tensionPerRun: 2.256 kN, ropeMassHanging: 40 kg, ropeRuns: 8, passes: no minimum entered
The same two ropes at 1:1 under EN: below the two-rope minimum of 16	factorOfSafety: 11.56, requiredFactor: 16, staticLoad: 10.38 kN, ropeRuns: 2, passes: no

For AI agents

This calculator is also the MCP tool elevator_rope_safety_factor at https://elevatorcalc.com/api/mcp, using the same function as this page. See how to connect, the llms.txt file, or the JSON catalogue.