The padstone calculator calculates average pressure over an area you enter. It does not select padstone dimensions, concrete strength or a mix. Those choices need a design of the complete bearing arrangement and the supporting structure.
Its useful role is to make the force, area and pressure units clear. A result in N/mm² is an arithmetic output, not an allowable stress or an approval to build.
Work through the units
Average pressure is force divided by area: p = R/A. Enter the reaction in kilonewtons and the two assumed contact dimensions in millimetres. The tool converts kilonewtons to newtons before dividing by the area.
For an invented example of 20 kN acting over an assumed 300 × 200 mm rectangle:
- Force: 20 × 1000 = 20,000 N.
- Area: 300 × 200 = 60,000 mm².
- Average pressure: 20,000 ÷ 60,000 = 0.333 N/mm², rounded.
The assumed rectangle is not a recommended padstone. Nor does entering those dimensions prove that the load actually acts over the whole area.
Change the assumed area and check the result
Keep the illustrative force at 20 kN and halve the entered area:
| Assumed contact rectangle | Area | Average pressure |
|---|---|---|
| 300 × 200 mm | 60,000 mm² | 0.333 N/mm² |
| 150 × 200 mm | 30,000 mm² | 0.667 N/mm² |
The average pressure doubles. This is a useful arithmetic check, but it does not say that either arrangement is suitable. Real contact, eccentricity and the spread of force through the bearing need to be established rather than assumed from the outside dimensions of a block.
Do not compare the output with an internet “safe masonry pressure” and stop there. Wall construction, material properties, geometry and the condition of the supporting structure are part of the design question.
Which force should be entered?
Use a reaction established for the situation being investigated, with its load basis recorded. A total load on a beam is not necessarily the reaction at one support. A service-load reaction is also not automatically interchangeable with a factored design action.
For the separate elementary case of a simply supported beam under a full-span uniform load, reactions are equal and each is wL/2. That symmetry does not apply to every load arrangement. Do not use this shortcut for an unseen building or for arbitrary point-load positions.
The load calculator guide explains the service-load model used on RSJ.info. It does not provide a complete bearing design or calculate the capacity of the supporting wall.
Information for the engineer
Bring the current beam drawing, support reaction information if available, wall thickness and construction details, photographs and relevant previous drawings. Mark where the beam is intended to bear and what is known about the structure beneath it. Ask what investigation is needed for anything concealed or uncertain.
Request a detail that resolves the bearing arrangement and any specified material, preparation, packing, restraint and connection requirements. Confirm how the contractor should report conditions that differ from the design. Do not replace missing information with the calculator’s demonstration defaults.
For England, Approved Document A provides structural guidance. Confirm the approval requirements for the actual jurisdiction and project. Our methodology page lists the calculations the tool performs and the checks it omits.
Correction to the earlier tool
An earlier version recommended dimensions from a generic masonry strength and rounding rule. That did not establish a valid bearing design and has been removed. If you retained an old recommendation, have the actual arrangement checked against the issued structural design.
Structural work must be designed by a chartered structural engineer and receive the applicable Building Control approval. This educational article has not received an independent engineering review.