Floor Area Ratio Calculator uses FAR = gross floor area ÷ lot area to estimate build ratio, zoning capacity, limit used, lot coverage, and remaining areas from your lot, GFA, footprint, and FAR limit.
FAR Measures Volume, Not Coverage — And That Distinction Matters
Floor Area Ratio gets cited constantly in zoning discussions, but it’s frequently misread as a limit on how much ground a building covers. It isn’t. FAR controls the total buildable volume relative to lot size — a 1.5 FAR on a 10,000 sq ft lot permits 15,000 square feet of gross floor area, but says nothing about whether that gets built as one sprawling floor or three compact ones. This calculator separates those two concepts, computing both your FAR position and your lot coverage in the same run.
What the Calculator Actually Computes
Four numbers come out of a single calculation pass. The hero figure — your proposed FAR — is just gross floor area divided by lot area. Everything else flows from comparing that against the zoning maximum you enter.
The Formulas Used
Proposed FAR:
FAR = Proposed Gross Floor Area ÷ Total Lot Area
Zoning Capacity (the ceiling):
Maximum Allowed GFA = Lot Area × Max Allowed FAR
Lot Coverage:
Coverage % = Building Footprint ÷ Lot Area × 100
Development Potential (remaining floor area before hitting the FAR cap):
Remaining Capacity = (Lot Area × Max FAR) − Proposed GFA
The “Floor Equivalents” figure on the Development Potential card divides that remaining capacity by your footprint area. It answers a practical question: how many more full floors could this building gain, at its current footprint, before hitting the zoning ceiling? If that number is 1.4, you have room for one additional full floor and a partial one before a variance would be needed.
Lot area can be entered as square feet or acres (US), or square meters or hectares (metric). The tool converts acres using the standard 43,560 sq ft per acre factor, and hectares using 10,000 sq m per hectare. GFA and footprint are always entered in the base area unit — square feet or square meters — regardless of which lot unit you choose.
Two Separate Constraints, One Tool
Zoning codes typically impose FAR and lot coverage as independent limits. A building can comply with FAR and still violate lot coverage — a single-story structure with a massive footprint could use its full FAR allowance in one floor while covering 80% of the lot in a zone that caps coverage at 40%. The reverse is equally common: a tall, slender tower might cover only 20% of the lot while exceeding the FAR limit through sheer floor count.
This is why the tool outputs both numbers simultaneously rather than treating them as the same calculation. If the FAR status shows Compliant but your lot coverage is unexpectedly high, that’s a red flag worth checking against your local code’s coverage maximum — which this tool does not enforce, because coverage limits vary too widely to embed as a fixed threshold.
A Worked Example
Mixed-use infill project on a 0.25-acre urban lot, zoned at FAR 2.0. The developer proposes a four-story building with a 2,700 sq ft footprint and 10,800 sq ft of gross floor area.
Entering the lot as 0.25 acres converts to 10,890 sq ft. Zoning capacity: 10,890 × 2.0 = 21,780 sq ft. Proposed FAR: 10,800 ÷ 10,890 = 0.99 — well inside the limit, using only about 50% of what zoning allows. Lot coverage: 2,700 ÷ 10,890 = 24.8%. Remaining capacity: 21,780 − 10,800 = 10,980 sq ft, which at the existing footprint represents roughly 4.1 additional floor equivalents.
In this scenario the FAR envelope is barely touched. The developer could add four more floors at the same footprint before FAR becomes a binding constraint — the practical limit here is more likely height restrictions or parking requirements than FAR itself. That’s exactly the kind of insight the Development Potential card is built to surface.
Frequently Asked Questions
If I switch between US and Metric, do my GFA and footprint values get converted automatically?
No. Switching the measurement system updates the unit labels on the GFA and footprint fields, but does not convert the numbers you’ve entered. If you’ve entered 1,000 sq ft and switch to metric, the field will show 1,000 sq m — a completely different quantity. Re-enter your values in the correct unit after switching systems to avoid incorrect results.
Why does “Floor Equivalents” show N/A instead of a number?
Three conditions cause this. First, if the proposed GFA already exceeds the FAR limit, there’s no remaining capacity to express as floors, so the field shows N/A rather than a misleading negative. Second, if the footprint is entered as zero, the division required to calculate floor equivalents is undefined. Third, if the footprint value exceeds the total lot area, the tool flags a footprint warning and suppresses the floor equivalents output as unreliable. Correct the footprint or GFA input to restore the result.
My lot is listed in acres on the deed. Does entering it as acres give the same result as converting manually?
Yes. Selecting “Acres” from the lot unit dropdown triggers a multiplication by 43,560 before any FAR calculation runs. Entering 0.5 acres produces the same lot base of 21,780 sq ft as typing 21,780 in the square feet field. The conversion is handled internally and does not appear in a visible intermediate step, but the Zoning Capacity card will display the converted square footage so you can verify it.
Can I enter zero for the proposed GFA to find out what the maximum allowed area is without a specific design in mind?
Yes. The GFA field accepts zero, and the tool will calculate Zoning Capacity and lot coverage normally. The FAR result will display as 0.00 and utilization will show 0%, but the Zoning Capacity card will correctly show the maximum allowable floor area for the lot and FAR you’ve entered. It’s a valid way to use the tool purely as a zoning envelope lookup.