Take the total square feet of the space, then multiply by the amount the landlord is charging per square foot (for example 1000 square feet X $5.00 per square foot = $5,000.00.) This is the total for the space for a year, now divide the number by 12 and you have the amount per month, 5,000.00 divided by12 = 416.67 per month. However do not forget to add CAM (common area maintenance) utilities etc, sometimes not included in the base rent.