Just use a laser rangefinder. If you can't borrow one, they are really cheap and make room measurements a trivial task.

I measured my flat in three steps:

1) I took a lot of photos - every space from multiple directions;

2) For each photo, I manually drew the "wireframe" of the space in Inkscape on a separate layer on top of the photo - wall/floor lines, doorways, windows, furniture;

3) I printed out the wireframe layers, grabbed a pencil and a measuring tape, and started to measure.

Step 2) could probably be semi-automated with some edge detection. Step 3) could probably be done if I had any recent experience doing photogrammetry. But fully manual solution wasn't that bad; I did my measurements over the course of a month, and finally made up a full floorplan in SweetHome3D with accurately sized furniture in one evening. This let my wife and me design a new furniture arrangement and buy appropriately sized movables in a single hour.

For people who, like me, lack the "mind's eye" and spatial imagination, having the flat digitized like this has immense value.

> Step 2) could probably be semi-automated with some edge detection.

All 3 steps can be done automatically with magicplan.

However once you get the results you'll probably need to manually measure and adjust because it's not quite accurate enough. But it's a really good start.

Wow, that's the exact app I wish I had then. But it'll come in handy, I have a new place to measure up pretty soon, thanks!

A simple tape measure will work fine. But make sure not to forget about wall thickness.

For an exterior an inch or 2 of accuracy is fine, but for interiors you should try to be accurate to around 1/4 or 1/8 inch (.5 cm).

It doesn't take as long as you might think, especially if you have a helper.

Make a rough floor plan (on paper), then measure and mark each wall on the plan.

Then enter all that into the program, and just stare at it - make sure it looks right. (It's far too easy to mess up, or not do the wall thickness right.)

Then go back and measure the location of doors and windows (including height, and offset from the floor). Put all that in.

Resist the urge to put in furniture until you are mostly done.

If you have multiple levels things will be more complex - you will need to know floor thickness, and ceiling height.

I got as many blueprints as I could of past renovations (my home is from 1938 and has seen major renovations multiple times). This was useful to get an idea of e.g. exterior wall thickness and to have some reference to validate measurements against.

I got a big A2 piece of paper to make a precise drawing at about 1:25 scale of the major walls. When dealing with things like wall thickness, I found that much easier than trying to enter my measurements directly in SweetHome3D, especially because I had never used it before, and the workflow takes some time getting used to.

I personally used this app to create the whole floorplan - https://www.magicplan.app/en/

Then I just double checked wall length with a tape measure, because I wanted 100% exact measurements, out of the box the app was 95% accurate. In my experience this is more than enough for the purposes of furniture placement planning and things like that, and it's pretty fast to do too.

You can get 'As-Builts' made, we did this recently in preparation for some remodel work - often people looking to break into home design will look to do them. I would check with home designers in your area and see if they have anyone they recommend.

Also check with your city - blueprints are on file going quite far back in many places.

Download a grid paper template and use a tape measure.