Square Footage Calculator for Floor Plans

Upload any to-scale floor plan image, click one wall you know the length of, and get accurate square footage in under two minutes. Works with CubiCasa, Matterport, iGUIDE, architect drawings, and any other to-scale plan.

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What makes a good floor plan for square footage calculation

Not all floor plans are equal. The key requirement is that the plan is drawn to scale, meaning spatial relationships in the image accurately reflect real-world dimensions. A plan where one inch equals eight feet throughout will give you accurate results. A hand-sketched rough layout where rooms are approximated will not.

Plans that work reliably:

CubiCasa, LiDAR-scanned, to-scale, widely used in Bay Area listings
Matterport, 3D tour floor plans, highly accurate
iGUIDE, photogrammetry-based, common in Canadian listings
Architect drawings, typically include a scale bar or explicit scale ratio
Appraiser sketches, any to-scale sketch exported as an image

Plans that will not work: non-scale diagrams, real estate marketing illustrations drawn for visual appeal rather than accuracy, and AI-generated floor plan images.

How the calculation works

PlanSnapper measures square footage by tracing the exterior perimeter of the structure and calculating the enclosed area. This is the correct method under ANSI Z765, the standard used by appraisers and lenders.

The process in three steps:

Upload the floor plan. Drag and drop any floor plan image. PlanSnapper uses computer vision (OpenCV.js) to auto-detect the exterior perimeter. For complex plans, use the manual trace tool to click along the exterior walls.
Set the scale. Click the two endpoints of any wall with a known measurement. Even one room dimension from the plan is enough. PlanSnapper calculates the pixel-to-foot ratio and applies it to the entire drawing.
Get your numbers. Hit Calculate. You get total square footage, individual wall lengths, and perimeter. Add a separate polygon for a garage, ADU, or any outbuilding that should be measured separately.

Why not just add up the room dimensions

It is tempting to multiply length by width for each room and add the results. This produces an inaccurate figure for several reasons:

Wall thickness is excluded. Room dimensions are typically interior measurements. The ANSI standard requires exterior measurement, which includes wall thickness. On a standard wood-frame house, this difference can be 3-6% of total GLA.
Hallways and closets are missed. Many floor plans show primary room dimensions but not hallway or closet dimensions. These spaces are part of GLA.
Irregular geometry is hard to account for. Bay windows, angled walls, and L-shaped rooms require additional geometric calculations when summing individual rooms. Perimeter tracing handles all of this automatically.

Privacy: your floor plan never leaves your device

PlanSnapper runs entirely in your browser using WebAssembly. The floor plan image is processed locally using OpenCV.js. No image data is sent to any server at any point during the calculation.

This matters for real estate professionals who work with client properties. Floor plans can contain sensitive layout information. With PlanSnapper, you have the same privacy guarantee as a desktop application without the installation overhead.

Multi-story homes and garages

For multi-story homes, run the calculation separately for each floor level and sum the results. Each floor plan page from CubiCasa or Matterport covers one level. Under ANSI Z765, each above-grade finished level contributes to GLA.

For attached garages, use the "Add Separate Polygon" feature. Draw a separate polygon around the garage footprint. Results will show living area and garage area as distinct figures, which is the correct reporting format for appraisals.

Compared to manual measurement

Method	Accuracy	Site Visit Needed?	Best For
Floor plan calculator (PlanSnapper)	High — ~1–2% on LiDAR plans	No	Desk review, pre-listing check, retrospective
Laser exterior measurement (ANSI Z765)	Highest — field standard	Yes	Primary appraisal; lender submission
Room-by-room sum (interior tape)	Lower — misses walls + hallways	Yes	Rough estimate only
Google Maps / satellite	Low — 10–20% off	No	Not recommended for GLA

Physical measurement with a laser device is the gold standard for a primary appraisal. But floor plan calculation is faster, does not require a site visit, and is reliable enough for desk reviews, retrospective appraisals, and pre-listing verification.

For buyers and agents, it provides an independent check on listed square footage before making or accepting an offer. On a $1M transaction, a 5% square footage discrepancy represents $50,000 in value. The two minutes it takes to verify is almost always worth it.

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Frequently Asked Questions

How do you use a floor plan to calculate total square footage?

Identify the scale of the floor plan, measure each room's dimensions using the scale, calculate area for each room (length times width for rectangles), and sum all rooms. For GLA, exclude garages, porches, basements, and unheated spaces. Tools like PlanSnapper let you trace a floor plan image and get automatic calculations.

What is the most accurate way to calculate square footage from a floor plan?

Trace the exterior perimeter of the above-grade living areas using the floor plan scale, then calculate the enclosed area. This mirrors the ANSI Z765 exterior measurement method and is more accurate than summing individual room measurements, which can miss hallways and wall thicknesses.

Can I trust floor plan square footage from a listing brochure?

Not always. Builder and listing floor plans are often marketing documents that may not be drawn to scale, may use different measurement methods, or may include spaces that do not qualify as GLA. Always verify against an appraiser's measurement or use a tool to trace the actual plan dimensions.

What does "to-scale" mean for a floor plan?

A to-scale floor plan means that spatial relationships in the drawing accurately reflect real-world dimensions throughout. If one inch on the plan equals eight feet in reality, that ratio holds for every room and wall on the page. CubiCasa, Matterport, and architect drawings are generally to-scale. Real estate marketing illustrations and rough sketches often are not.

Can I calculate square footage from a PDF floor plan?

Yes. Export or screenshot the PDF floor plan as an image (PNG or JPG), then upload it to a tool like PlanSnapper. Set the scale by clicking two endpoints of any wall with a known measurement, and the calculator handles the rest. Most architect and CubiCasa PDFs include labeled wall dimensions you can use to set scale.

How do I set the scale in a floor plan calculator?

Click two endpoints of any wall or room dimension you know the real length of - for example, a 12-foot wall labeled on the plan. Enter that measurement when prompted. The tool calculates the pixel-to-foot ratio and applies it to the entire drawing. One accurate calibration measurement is all you need.

Does a floor plan calculator work for multi-story homes?

Yes. For multi-story homes, run the calculation separately for each floor level using the corresponding floor plan image. Sum the above-grade finished levels for total GLA. Most CubiCasa and Matterport exports include a separate plan for each level. Exclude basement levels from GLA and report them separately per ANSI Z765.

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More guides on floor plan measurement tools:

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More guides on measuring square footage:

← Back to: Floor Plan Measurement Tools: The Complete Comparison Guide