This is the most common specification error in the category and it is entirely avoidable with one measurement and one multiplication.

Measure the gap, not the post

A belt is specified by the distance between the two posts it spans, plus allowance. The post’s own dimensions are irrelevant to it.

Measure the actual gap on the floor where the barrier will stand, not the gap on the plan — posts get placed around door swings, columns and floor boxes, and the real span is usually a little wider than the drawing.

Add about a fifth

A belt should reach comfortably with webbing still in the cassette. Roughly a fifth longer than the gap is the working rule.

A belt at full extension will not reach if a post is nudged, pulls the posts inwards, and puts the load into the cassette mechanism rather than the webbing. It is also the state in which a belt is most likely to unclip.

Different legs need different belts

A switchback almost never has uniform gaps. Order uniform belt lengths for it and you will have three that are too short and two that trail on the floor.

Send the floor plan and get a layout drawing with a belt length per leg. It is free and it takes twenty minutes at our end.

Know when to add a post instead

Past about nine metres a belt sags visibly and the recoil becomes hard to brake safely.

At that point the answer is another post, not a longer belt, and any supplier who sells you fifteen metres of webbing is not doing you a favour.