Metal is not ordered by area. It is ordered as a specific number of panels at a specific length, which makes accuracy matter far more here than it does with shingles.
Shingles are small units cut from a pile, so a measurement that is a few inches out disappears into the waste allowance. Metal panels are cut to your stated length at the factory or on a roll former, and a panel cut short is scrap.
That changes what you measure and how carefully. You are producing a cut list rather than an area, and the numbers that matter are the panel run and the coverage width.
A panel is wider than the roof it covers, because the edges overlap or interlock with the next panel. A 36 inch panel typically gives 36 inches of coverage with the overlap already accounted for, but some profiles differ. Always calculate on the stated coverage width, never on the physical width.
Panel length: eave to ridge, along the slope. Not the horizontal run. This is the actual sloped distance a panel has to span, and it is the single most important number on the job.
If you can measure it directly on the roof, do. If not, calculate it: the horizontal run multiplied by the pitch factor gives the slope length. A 15 foot run at 6/12 pitch is 15 times 1.118, which is 16 ft 9 in. The pitch factor table is here.
Add your eave overhang, usually an inch or two past the drip edge so water throws clear, and check the manufacturer's specification for the figure they expect.
Roof width at the eave, for each plane. Divide this by the panel coverage width to get the number of panels. Round up, always.
Each plane separately. A hip roof has trapezoidal and triangular planes where panels are different lengths across the plane. Every panel on a triangular hip section is a different length, and that becomes a cut list rather than a single number.
Eave width divided by coverage width, rounded up. A 32 foot wide plane with 36 inch coverage panels needs 32 divided by 3, which is 10.67, so 11 panels.
That last partial panel is normal and unavoidable. What is worth checking is where the cut lands: a final panel only a few inches wide is awkward to fix and looks wrong. Many installers adjust by trimming the first and last panels equally so both edges are balanced, which is a decision to make before ordering rather than on the roof.
One of metal's advantages is running a single panel from eave to ridge with no horizontal joint. On a long slope that stops being practical.
Very long panels are difficult to transport, awkward to lift in wind, and prone to oil canning if handled badly. Beyond roughly 20 to 25 feet it is worth discussing with the supplier whether to split the run and use a transverse lap, which then needs its own overlap allowance added to the total length.
Thermal movement matters too. Metal expands and contracts significantly along its length, and long panels need fastening systems that allow it. This is a specification question rather than a measuring one, but the panel length you choose triggers it.
Metal roofs need a lot of formed trim, all of it measured linearly and all of it easy to under-order.
Trim comes in fixed lengths, commonly 10 feet, and laps at each joint. Divide your run by the piece length, round up, and allow for the lap. Rake trim in particular is measured along the slope, and using the horizontal measurement is a common way to come up short.
The eave widths and the building footprint are straightforward ground level readings. Slope length can be derived from the run and the pitch without going up, which is the safer route.
Where it becomes awkward is that metal roofs are bright and reflective. A laser aimed at a light coloured metal surface in sun is close to worst case: the beam scatters poorly off the shiny finish and the dot is invisible. If you are measuring on an existing metal roof, expect to need a target plate at the far end, or a unit with a camera viewfinder. The reason reflective surfaces defeat these tools is worth understanding.
Given panels are cut to order, it is worth having someone else check your figures before the order goes in. The full roof method is here.
Working out the right laser measure for the job in front of you is what our front page is for.