--- title: "Floor area ratio" description: "What floor area ratio is, how to calculate it, and how it differs from lot coverage and gross floor area, with a Toronto lot and a Chicago district example." last_updated: '2026-10-10' --- # Floor area ratio Floor area ratio is counted floor area divided by lot area. A ratio of 0.6 allows counted floor area equal to 0.6 times the lot. Canadian by-laws often call the same measure floor space index or floor space ratio. The by-law decides which floor area counts, and the maximum usually changes by zone. ## The formula, with two worked examples Floor area ratio equals counted floor area divided by lot area. Rearranged, counted floor area equals lot area multiplied by the ratio. The by-law names the floor area that counts. Basements, parking, balconies and mechanical rooms are often excluded or counted in part. Multiplying the two numbers does not apply those rules. The Toronto row is 29 Wychwood Ave. The Chicago row uses the RS3 maximum in the zoning ordinance and an example lot area. It is not a lookup of a named parcel. Worked floor-area examples. The Chicago lot area is an example input. | Example | Lot area | Maximum ratio | Counted floor area | What the figure does not decide | | --- | --- | --- | --- | --- | | 29 Wychwood Ave, Toronto | 162.06 m² | [0.6 — By-law 569-2013, 10.10.40.40(1)(A)](https://www.toronto.ca/zoning/bylaw_amendments/ZBL_NewProvision_Chapter10_10.htm) | 97.236 m² | The zone label is R (d0.6). Regulations (1)(C) and (1)(D) remove this limit for some buildings. The parcel area is from City of Toronto Property Boundaries. | | Example lot in Chicago RS3 | 3,125 sq ft | [0.90 — Zoning Ordinance 17-2-0304-A](https://codelibrary.amlegal.com/codes/chicago/latest/chicago_il/0-0-0-103109) | 2,812.5 sq ft | Section 17-17-0305 governs how floor area is measured. Section 17-2-0304-D can leave some ground-floor area out of the ratio in RS3. | - [City of Toronto, Property Boundaries](https://open.toronto.ca/dataset/property-boundaries/) - [Zoning By-law 569-2013, Section 10.10 (R zone)](https://www.toronto.ca/zoning/bylaw_amendments/ZBL_NewProvision_Chapter10_10.htm) - [Chicago Zoning Ordinance, Title 17](https://codelibrary.amlegal.com/codes/chicago/latest/chicago_il/0-0-0-103109) ## Floor area ratio, lot coverage and gross floor area These three measures answer different questions. A lot can satisfy one and fail another. How floor area ratio, lot coverage and gross floor area differ | Measure | Question it answers | What it leaves out | | --- | --- | --- | | Floor area ratio | How much counted floor area the lot may hold, as a multiple of the lot area. | The by-law exclusions, until you apply them. Upper storeys do count. | | Lot coverage | How much of the lot the building footprint may cover. | Upper storeys. A second floor adds floor area and does not add footprint. | | Gross floor area | The floor area the by-law counts, in square metres or square feet. | It is an area, not a ratio. The same gross floor area is a different ratio on a different lot. | - [Setbacks and lot coverage](https://buckybuild.com/en/setbacks) - [Highest and best use](https://buckybuild.com/en/highest-and-best-use) - [Development charges in Canada](https://buckybuild.com/en/development-charges) ## How the maximum changes by zone The maximum is a zone rule, not one number for a city. Toronto writes floor space index into the zone label on the Zoning By-law Map. Chicago publishes a table by district, and some districts allow a premium on top of the table. The calculator on this page multiplies a lot area by a ratio. It does not read a zone, and it does not apply exclusions or premiums. The same kind of limit, set differently by zone | Place | Where the maximum is set | Figure | | --- | --- | --- | | Toronto, R zone at 29 Wychwood Ave | [By-law 569-2013, 10.10.40.40(1)(A)](https://www.toronto.ca/zoning/bylaw_amendments/ZBL_NewProvision_Chapter10_10.htm) | 0.6, from the d0.6 on the zone label | | Chicago, RS3 | [Zoning Ordinance 17-2-0304-A](https://codelibrary.amlegal.com/codes/chicago/latest/chicago_il/0-0-0-103109) | 0.90 | | Chicago, RM6 | [Zoning Ordinance 17-2-0304-A](https://codelibrary.amlegal.com/codes/chicago/latest/chicago_il/0-0-0-103109) | 4.40 before any premium. Section 17-2-0304-C may add up to 25 percent. | - [Glossary](https://buckybuild.com/en/glossary) - [Methodology](https://buckybuild.com/en/methodology) - [Coverage](https://buckybuild.com/en/coverage) ## Common questions ### How do you calculate floor area ratio? Divide the floor area the by-law counts by the lot area. To go the other way, multiply the lot area by the maximum ratio. Then apply the by-law’s exclusions, exceptions and premiums. The product alone is not the permitted building. ### Is floor area ratio the same as floor space index? They are the same kind of measure. Floor area ratio is the usual United States term. Toronto’s Zoning By-law 569-2013 uses floor space index. Floor space ratio is another name for it. Read the local definition before comparing a number across cities. ### What is the difference between floor area ratio and lot coverage? Floor area ratio compares counted floor area with lot area, so upper storeys count. Lot coverage compares the building footprint with lot area, so upper storeys do not. A lot can pass one limit and fail the other. ### Does the calculator on this page apply my city’s exclusions? No. It multiplies the lot area and the ratio you enter. Exclusions, premiums and zone exceptions stay in the by-law. Check an address to see the ratio on file for a live municipality, with the section citation. ## Sources - [City of Toronto, Property Boundaries](https://open.toronto.ca/dataset/property-boundaries/) - [Zoning By-law 569-2013, Section 10.10 (R zone)](https://www.toronto.ca/zoning/bylaw_amendments/ZBL_NewProvision_Chapter10_10.htm) - [Chicago Zoning Ordinance, Title 17](https://codelibrary.amlegal.com/codes/chicago/latest/chicago_il/0-0-0-103109) [Glossary](https://buckybuild.com/en/glossary) · [Coverage](https://buckybuild.com/en/coverage) · [Methodology](https://buckybuild.com/en/methodology) · [MCP](https://buckybuild.com/en/mcp)