Comment mesurer la surface au sol avec votre iPhone (pour locataires et acheteurs)

A study by ratemyagent.com.au in 2023 found that 27% of property listings overstate square footage by more than 5%. For a 60 m² apartment, that's 3 m² of imaginary space, about the size of a small bathroom. If you're renting at €25/m²/month, you're being asked to pay €75/month for floor that doesn't exist.

Here's the 90-second method to verify a listing before you sign.

The fast method (with LiDAR)

On any iPhone Pro from 12 onwards (or iPad Pro 2020+):

1. Open Ruler AR and tap Room Scan.
2. Stand in one corner. Walk slowly along each wall.
3. When you return to the starting corner, tap Done.
4. Read the floor area from the summary screen, accurate to within ±2%.

This takes about 60 seconds for a single room. A full one-bedroom apartment takes 3-4 minutes total.

The reliable method (any iPhone)

If you don't have a LiDAR iPhone, use the Manual Room Builder:

1. Open Ruler AR → Tools → Manual Room Builder.
2. Stand in a corner, aim the crosshair at the floor at your feet, tap.
3. Walk to the next corner along the wall. Tap.
4. Continue around the room. The mini-map shows the polygon growing.
5. When you reach the starting corner, the "close room" pulse appears. Tap it.

The polygon's area is computed automatically using the Shoelace formula. Accuracy depends on AR tracking, typically ±5% for rooms under 30 m², which is good enough to catch listing misrepresentations.

What counts as "square footage", and what doesn't

Listings inflate numbers in three common ways:

• Including unusable spaces. Sloped ceilings, alcoves under 1.5 m high, structural columns. Different jurisdictions handle these differently, UK/AU typically exclude under-1.5m areas; US is laxer.
• Measuring exterior wall-to-wall instead of interior. A 10 × 10 m exterior box is only ~9.5 × 9.5 m of usable interior space after walls (5 cm of internal partition × ~6 walls = 30 cm of "ghost" wall area).
• Including balconies/terraces at full count. Many places count outdoor space at 50%, if your listing counts it at 100%, you're getting half what you think.

When you measure with the app, you're measuring interior usable floor area. That's the most honest number, and the one that matters for furniture planning and pricing comparisons.

What to do when the numbers don't match

If your measurement is more than 5% below the listed area, you have leverage:

• Renting: Ask for a reduction proportional to the discrepancy. €/m² is a transparent calculation, if you're paying for 60 m² and the apartment is 56 m², that's 6.7% you can push for.
• Buying: Get a professional surveyor before you sign anything. A 5% area discrepancy on a €400,000 purchase is €20,000, that's worth the €500 surveyor fee.
• Insurance: Wrong square footage means wrong coverage. Insurers will pay claims based on actual measured area if you have documentation. The app's PDF export with date stamp becomes evidence.

Multi-room measurement

For full apartments, save each room as a separate project. Total area = sum of all rooms minus the overlap (hallway shouldn't be counted twice if you measured it as part of the entrance and the kitchen). The PDF export totals it automatically when you group projects under one folder.

Convert between units

The app handles all major units: m², ft², cm², in². Switch in Settings → Units. The PDF export labels everything in your chosen unit. If you're in the US shopping for a European apartment (or vice versa), this saves you the conversion math.

Bottom line

Never trust a square-footage number you didn't measure yourself, especially on a rental or purchase listing. The app gives you a 90-second verification that's accurate within a few percent and produces documentation you can use to negotiate. Treat it like a spell-check for real estate listings.