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Understanding Rental Space Measurements

Updated: Mar 3, 2023

Written by Ross Rains

There are three ways to think about office space.

Imagine you had a space with inside measurements of 30 ft. by 20 ft. That would come to 600 sf INSIDE MEASUREMENT.

But, don’t forget the walls needed to define and protect your space! An outer wall could be one foot thick. Inner walls are typically 6” thick. The BOMA standard (an International Measurement standard organization) says that half the all thickness between tenants relates to each tenant.

That would add 30 + (20 + 30 + 20)/2 = 65 sf just to the wall component, totalling now to 665 sf This is called the USABLE area.

However, recall that there are lobbies, halls, washrooms, mechanical rooms, stairwells, maintenance rooms, etc. which all need to be maintained, insured and serviced by the landlord, not to mention city property taxes that apply. This is called COMMON AREA and is shared proportionately among all tenants. The common area in buildings can be 11 to 40+% depending on the shape and structure of a building. Each of these common area components are a key part of the presentation and support of your new desired space!

So, if the common area factor is 35%, that 665 sf USABLE space would then be called 897.65 RENTABLE space. Remember, the greater the common area, the greater the presentation and amenities which serve you and your clients ... Landlords advertise the RENTABLE space, not the USABLE space. In this case, the Landlord would advertise 898 sf and assign a flat rate or sf value to the space. That rate would be a

function of term, additional changes needed to suit, extras included such as parking, etc.

This is why it is difficult to quote space by phone or email.

Always visit and ‘get the whole picture’ from which a written quote will then have context and accuracy, so that you can determine the right space, in the right building, at the right size, with the right landlord at the right price! ☺


Ross Rains


Ross Rains leads a family run business that provides office facilities to over 100+ businesses and professional in the London – St. Thomas area, through 100,000+ sf of owned commercial office facilities. He is also the President of Tier1 Exec Corporation and coaches Entrepreneurs and CEOs in how to take their businesses to the next level.

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