Why Office Chairs Have Five Legs Instead of Four

Emily Zhang of Rabbit Hole explained why a typical office chair has five legs instead of the standard four legs found on other chairs. So, in engineering terms, more legs equals more stability.

The math is clear. For a given seat size, for more legs the chair has, the less likely it is to tip over. This is the basic principle behind why all office chairs have five legs. But it can’t be the full answer. Because if more legs are always better, why stop at five? Why not six, seven?

Zhang’s calculations show that the largest stability ratio becomes smaller after four chair legs.

…the largest stability improvement is from 3 to 4 legs. The rest are marginal. In fact, they all add more material than they do stability. But to go from 3 to 4 legs, you only add 33% more material and get 41% more stability.

This theory was confirmed with the introduction of the iconic Aeron Chair from designer Herman Miller

Chair design was evolving into a science. This was accelerated by a new US trade association called BIFMA, whose goal was to wrangle hundreds of new office furniture designs into a set of universal safety standards. Right from the 70s and still to this day, they focused on stability. But if office chairs are now a science, all of their fundamental laws are rewritten in 1994 with the Aeron Chair.

Zhang also referred to BIFMA (Business and Institutional Furniture Manufacturers) as to their stance on five-legged office chairs.

The BIFMA standards don’t require five star bases. Instead of modeling some optimal number of legs, They did the smart, simple solution. Can chair hold weight far back? Their official stability standard is just a field test. Can the chair support a stack of weights leaned all the way back, as well as weights perched on the very front of the seat. It’s just that these tests are very hard to pass with legs of reasonable length until you go up to five.

The BIFMA test was not easy on the chairs, thus proving that five-leg chairs offer the best stability for office chairs.

By the way, BIFMA standards go hard. Dropping a weight bag 100,000 times to test durability?

Office Chair Five Legs

via Tom Scott

Lori Dorn
Lori Dorn

Lori is a Laughing Squid Contributing Editor based in New York City who has been writing blog posts for over a decade. She also enjoys making jewelry, playing guitar, taking photos and mixing craft cocktails.