Area gets you most of the way. The rest is linear: every eave needs a starter course, every ridge and hip needs cap, and every valley eats material at an angle. Those never appear in a square footage figure.
Measure the footprint including overhangs, apply the pitch factor, and divide by 100. That gives you roofing squares, which is how everything here is priced. The area calculation and the pitch multiplier table are here.
Most architectural shingles are packed three bundles to the square, though this varies by product and weight, so check the wrapper rather than assuming. Heavier designer shingles sometimes run four or five bundles per square.
Walk the roof outline on your sketch and total up four separate lengths. Each one consumes material that no area calculation accounts for.
Eaves. The bottom edge of every roof plane needs a starter course, a strip laid before the first full course so the joints are sealed and the wind cannot lift the first row. Measure the total length of every eave.
Rakes. The sloping edges at gable ends. Many manufacturers now specify starter along the rakes as well as the eaves for wind resistance, so measure these too and check the product specification.
Ridges and hips. Both need cap shingles, which are either purpose made or cut from three tab shingles. Measure the total run of every ridge and every hip.
Valleys. Where two planes meet, shingles are cut at an angle and the offcuts are waste. Measure the total valley length, because it drives your waste allowance more than anything else on the roof.
Every course running into a valley is cut on a diagonal, and the cut piece is rarely usable elsewhere. A roof with several valleys can easily need double the waste allowance of a plain gable roof of identical area.
Roofing waste is driven by roof complexity rather than by material choice, which is the opposite of flooring.
A simple gable roof, two planes and one ridge, carries a modest allowance because almost every shingle is laid full and the only cuts are at the rakes. A hip roof with several valleys, dormers and penetrations carries a much larger one, because a high proportion of shingles are cut.
Count the features on your sketch. Two planes and a ridge is simple. Four or more planes with valleys, or any dormers, is complex. Suppliers will advise on the allowance for the specific product, and it is worth asking rather than applying a generic number, because it varies with shingle format as well as roof shape.
Shingles are one line on the order. These are measured at the same time and forgotten just as often.
Everything above assumes you can measure the planes. Frequently you cannot, or should not.
From the ground, measure the footprint and derive the pitch, then calculate. The linear runs are harder but most are measurable from below: eave lengths are the building's outside dimensions, rake lengths follow from the pitch and the run, and ridge length is usually the building length minus any hips.
Valleys are the genuinely difficult one from the ground, and they are also the biggest driver of waste. If the roof has several, that is the case for either getting up safely or having a roofer quote it.
A laser measure with indirect mode covers most of the ground based work, and on a bright roof the limiting factor is seeing the dot rather than the tool's range. The roofing picks are here, and indirect height measurement is covered here.
Two sanity checks. Does your square count look sensible against the building size, roughly the footprint in hundreds of square feet plus the pitch uplift? And have you priced starter, cap and underlayment as separate lines rather than assuming the shingle count covers them?
Ordering short is far worse than ordering long here, because shingles vary between production batches and a colour shift across a roof is visible from the street and cannot be fixed without stripping it. The full roof method is here.
If you have arrived at the wrong page for what you need, start here instead.