The four measurements that settle an order before you place it

What you need before you start

A tape measure, a pen, and somewhere to write four numbers that you will still be able to find in six months. A phone note works. So does a photograph of the tape across each measurement, which has the advantage of being unambiguous when you come back to it.

Take all four in one sitting. Taken separately they drift, and it is the relationship between them — your body against a bag, your bag against a mat, a mat against a floor, a floor against a pitch — that actually decides what to buy.

1. Your lying length, and the bag length beside it

Stand against a wall and measure your height. Write it down.

Then go to the specification rather than the marketing. A sleeping bag is published with a stated length and normally a shoulder girth, and both are internal figures for the bag rather than statements about who fits in it. The rating on the label assumes a certain amount of dead air around you: too short and the insulation is compressed at the feet, too generous and there is more air to warm than the test assumed. The foot box shape is part of the same question and rarely photographs usefully.

If a bag quotes comfort and limit figures under EN 13537 or ISO 23537, those come from a heated mannequin in a laboratory. They are an indication for comparing one bag against another, not a promise about a night, and the fit you have just measured is one of the things the test holds constant and real life does not.

If you sleep in a hammock, the equivalent measurement is length, and it is the figure most listings bury. A gathered-end hammock is slept in diagonally rather than along its axis, so the fabric has to run well past your own height before the flat diagonal lie exists at all. Width then sets how far across that diagonal you can move.

2. Shoulder width, for the mat

Sit on a chair, measure across your shoulders, and add nothing.

Now read the mat’s width and length. Regular sizes leave broad shoulders on the cold floor, and that is not a comfort problem but a warmth one: the R-value the maker publishes describes the mat, and a shoulder resting on the ground is not on the mat. Buy the widest one the load you carry will tolerate.

Take thickness at the same time, because it answers a separate question — whether a side sleeper’s hip reaches the ground through a compressed mat. And check the R-value is quoted against ASTM F3340, the common test that finally made figures from different makers comparable, rather than an in-house number.

One useful consequence of the standard: R-values add up. A closed-cell foam mat under an inflatable one gives you the sum, which is a cheaper way of changing the figure than buying a second mat.

3. The pitch: floor dimensions, and what goes under them

This one is measured on paper rather than on your body, and it is where returns come from.

Take the tent’s floor area per person and its floor dimensions, then take its peak height and — the figure that matters more — how much of the length stays at full height before tapering into wall. A tent that is nominally long enough but tapers from halfway down is a tent you sit up in once.

Then measure the footprint, if you are buying one, against those same floor dimensions. The compatibility rule is counter-intuitive: a footprint has to be slightly smaller than the tent floor. One that sticks out past the flysheet collects rain and channels it underneath. Model-specific footprints are cut to the maker’s plan and share the tent’s peg points; if you buy or cut a universal sheet, take the floor dimensions from the specification and subtract 3 to 5 cm on every edge.

Finally, note the pitch type — freestanding, semi-freestanding, or pitched on trekking poles. This is not a dimension, but it is the constraint the dimensions sit inside: it decides whether the tent will stand on a wooden platform or needs ground soft enough to take a peg.

4. Packed length, measured against the space it has to go into

The last measurement is of a space, not an object.

If the kit is carried, measure the packed dimensions of the shelter, the mat and the bag, and compare them against what you already own. Note that dry bags reach their stated volume only with the closure open: a roll-top seals when the fabric is folded over at least three times before the buckle clips, so a 10 litre bag carries rather less than 10 litres of kit.

If the kit travels in a car, measure the boot floor once the tent is in, not when the car is empty. Packed length rules out more camp tables than any other figure, and two storage boxes that tile the width beat three that leave a gap you cannot use.

And if you fly or travel with poles, measure the bag they have to go inside before anything else. Packed length against stated maximum length is the whole trekking-pole decision: Z-fold poles pack shortest and adjust over the smallest range, lever-lock poles adjust over the widest and pack longest.

The measurement that decides a compatibility, not a fit

None of the four above will tell you whether a spare pole section, a gas canister or a filter cartridge will fit what you already own. Those are a different exercise with a different tool — callipers rather than a tape — and they are covered on the page about measuring for spares. Do not attempt them from a photograph, and do not attempt them from memory.

Write the four down somewhere permanent

Your height and shoulder width do not change. The floor dimensions of the tent you own do not change either. Kept together in one note, those four numbers answer almost every question a listing will ask you, and they turn an evening of guessing into a five-minute decision.