Several of these get you within a few percent, which is enough to decide whether a sofa fits or roughly price a job. None of them is good enough to cut material from, and it is worth being clear about which you need.
The most reliable approach is to find something nearby with a known dimension and count how many fit along the wall.
A sheet of drywall is exactly 4 feet wide, and in an unfinished or partly built room this is the best reference available. If the wall is already boarded, the joints are 4 feet apart.
A standard interior door is close to 80 inches tall and typically 30 to 36 inches wide, though width varies more than height. The height is the more dependable of the two.
A US dollar bill is 6.14 inches long, near enough to 6 inches for estimating. Credit cards are a standard 3.37 by 2.13 inches. Both are useful for short measurements because everyone has one.
Standard floor tiles are commonly 12 inches, and a laid tile floor gives you a ready-made ruler if you count the tiles and add the grout lines.
These are worth measuring properly on a day when you do have a tape, because then you carry a rough ruler with you permanently.
Arm span is approximately equal to your height for most people. Fingertip to fingertip with arms outstretched.
Pace length. Walk a known distance, count the paces, divide. Most adults land somewhere around two and a half feet per natural pace, but individual variation is large enough that assuming the average is worse than measuring your own.
Hand span, thumb tip to little finger tip, spread. Useful for short spans.
Foot length, which is genuinely handy because you can heel-to-toe along a wall and count.
Write these down somewhere. The value is entirely in having measured them once rather than guessing.
Walking paces vary with how fast you move and whether you are concentrating. Heel-to-toe steps do not, because each step is physically constrained by your own foot. It is slower and considerably more consistent.
Recent phones with depth sensors run measuring apps that use augmented reality to estimate distance. On newer hardware these are surprisingly decent for short indoor spans.
Be realistic about the limits. They degrade quickly in poor light, on plain untextured surfaces where the software has nothing to lock onto, and over longer distances. Treat the result as a good estimate rather than a measurement, and verify anything important.
They are genuinely useful for exactly the situation this page describes: standing in a shop wondering whether something will fit.
The distinction that matters is between planning and cutting.
Planning is fine with estimates. Will this fit through the door, roughly how much paint, is this room bigger than that one, will the sofa work against that wall. A few percent of error changes nothing.
Cutting is not. Anything where material gets shortened permanently needs a real measurement. A shelf, a worktop, a trim length, a curtain pole. An estimate here produces a piece that is wrong and cannot be un-cut.
The reason a laser measure is worth having is precisely that it collapses this distinction. It is fast enough to use for casual planning and accurate enough to cut from, and it does long spans single handed, which is the thing a tape genuinely cannot do. Measuring alone is covered here, and a capable one costs about twenty dollars.
If you are reading this because a tape is not to hand right now, the estimates above will do. If you are reading it because you do not own one, that is a different problem with a cheap solution. The proper wall measuring method is here.
Working out the right laser measure for the job in front of you is what our front page is for.