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Spiral stair calculators

Spiral stair calculator (narrow)

This calculator is based on Approved Document Part K 2010 which in turn uses BS 5395-2:1984

Enter your guestimate values into the green boxes and then the red text comments indicate whether the necessary conditions are met. Other values such as riser height and pitch get calculated. If the conditions are not met you need to try different goings, etc.

Bear in mind that the calculator works on clear width and does not take into account the extra width taken up by handrails and other guarding such as balusters or glazing

The radius is the radius of the outer (larger) curve on the stair. The width is the width of the treads.

Notes for above calculator

  1. Having more risers than the minimum may make for a less steep stairs. The risers will be smaller and the formula (2x riser + going) will allow for longer goings
  2. Controlling by the radius helps to fit the stair within a limited space whereas controlling by the amount of turn may be more important so that the landings are positioned correctly
  3. Adjusting the width affects where the centre line of the steps is and hence the pitch. Note that there is no minimum set width except that means of escape from fire and disabled access should both be considered. Bear in mind that the going on steps is measured to the edge of the nosing (see diagram 1 in section 1 of the Approved Documents part K)
  4. Adjusting the goings affects the pitch and the overall length of the stairs
  5. The formula (2x riser + going) is included in the building regulations and is intended to make for a comfortable and safe stride
  6. The pitch is measured along the centre line of the nosings
  7. If the width at the narrow end is too low you may be able to adjust this by increasing the radius of the stair, having longer goings or setting a lower stair width
  8. The total angle turned will affect the position of landings and which way you are facing when you reach the top (or bottom) of a flight. You may be able to make adjustments by having an extra piece of triangular landing to create the necessary amount of turn
  9. The 2000mm headroom is measured above the pitch line. It can be reduced slightly for certain loft conversion configurations
  10. Bear in mind that after you have calculated the stairs you will need to allow something extra in width for things like the thickness of handrails (and space for hand movement), balustrades, stair construction etc.

See more detail at Staircase Design


Spiral stair calculator (wide)

This calculator is based on Approved Document Part K 2010 which in turn uses BS 5395-2:1984

Enter your guestimate values into the green boxes and then the red text comments indicate whether the necessary conditions are met. Bear in mind that the calculator works on clear width and does not take into account the extra width taken up by handrails and other guarding such as balusters or glazing.

With spiral stairs of a metre or more in width things get quite complicated because the steps have to be checked in two places for steepness and size. (see diag. 8 above) This means there are effectively seven variables which can all affect each other. The best way is to simply play around with the calculator and get a feel of the dynamics. Values such as riser height and pitch also get calculated.

Notes for above calculator

  1. Having more risers than the minimum may make for a less steep stairs. The risers will be smaller and the formula (2x riser + going) will allow for longer goings. Note that with the riser dimension it is necessary to work to fractions of a millimetre since the cumulative effect of many stairs can make a large difference to the overall floor to floor dimension.
  2. Controlling by the radius helps to fit the stair within a limited space whereas controlling by the amount of turn may be more important so that the landings are positioned correctly
  3. Remember that the space needed for the stairs will be slightly larger than the radius because of handrail, banisters etc.
  4. This should be 1000mm or more. If you want it less then use the other calculator above
  5. Increasing the width has the effect of creating a larger difference between the two goings which get measured and this can cause the outer (2R+G) formula to exceed its limit.
  6. Adjusting the goings affects the pitch and the overall length of the stairs
  7. The (2x riser + inner going) formula comes from the Building Regulations and must be satisfied at both going measurement points (270mm in from the end of the steps). The pitch is also measured at both points
  8. The spreadsheet works by first setting the inner size of the step and then checking to see if the outer size is OK
  9. The total angle turned will affect the position of landings and which way you are facing when you reach the top (or bottom) of a flight. You may be able to make adjustments by having an extra piece of triangular landing to create the necessary amount of turn
  10. The 2000mm headroom is measured above the pitch line. It can be reduced slightly for certain loft conversion configurations
  11. The amount of turn may be a more important factor than the space available. If so use this part of the calculator (after first using the very top part where you input the floor to floor height and number of steps)
  12. Increasing the width has the effect of creating a larger difference between the two goings which get measured and this can cause the outer (2R+G) formula to exceed its limit.

See more detail at Staircase Design