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Capacity-to-Load Ratio & Factor of Safety

Calculate the ratio of positive load capacity to applied design load, including ratios at or below one.

  • Formula & worked example
  • Private in your browser
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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Private by default

Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

Formula and method

Capacity 1,000 N divided by load 500 N gives a ratio of 2. If load rises to 2,000 N, the ratio falls to 0.5 and the entered load exceeds the entered capacity.

Capacity-to-load ratio = capacity / load

Worked example

Enter these known values and leave the other values blank.

Load capacity for the chosen failure mode
1000 N
Applied design load
500 N
Capacity-to-load ratio
2

Assumptions and limitations

  • Capacity, load and ratio must be positive. A ratio below one means the entered load exceeds capacity; one means they are equal.
  • Compare capacity and load using consistent definitions for the same failure mode, loading direction, duration and conditions. Ultimate and yield capacities are different choices.
  • A ratio above one alone does not establish that a design is safe or compliant. Required factors, load combinations, uncertainty, fatigue, buckling and other failure modes require the applicable engineering assessment.
  • One kip is exactly 1,000 pound-force. The result is a capacity/load ratio, not a percentage or margin of safety.

Common questions

Why does the calculator show a value below one?

That is a valid ratio when entered load exceeds entered capacity. Hiding it would conceal the most important implication of those inputs.

Is factor of safety the same as margin of safety?

No. A margin often involves a ratio minus one and may also include a required factor. State the applicable definition before comparing the two.

References

Bookify allows every positive capacity-to-load ratio, including values at or below one that indicate no positive capacity margin. The pound-force uses the exact international pound and standard gravity instead of the rounded captured factor. One kip is exactly one thousand pound-force, using the international pound and standard gravity.

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