Four ways to measure it, three of which keep you on the ground. You need this number before you can order a single bundle of anything.
In everyday roofing use these are treated as the same thing, and for practical purposes they are. The number you want is expressed as rise over run: inches of vertical rise for every 12 inches of horizontal run.
A 6/12 roof rises six inches per foot. A 4/12 rises four. Written as a ratio it is 6:12, spoken as "six twelve", and occasionally converted to degrees, where 6/12 is about 26.6 degrees.
Strictly, engineers distinguish pitch as rise over total span and slope as rise over run, which produces different fractions. Roofers, suppliers and building inspectors almost universally use rise over 12, so that is the convention worth knowing. If someone hands you a pitch expressed as a fraction like 1/4, check which they mean before ordering anything.
Pitch converts the footprint you can measure into the roof area you actually cover. It also determines which materials are permitted at all, since most asphalt shingles have a minimum pitch below which they are not rated.
The safest method and the most accurate of the ground based ones.
Stand back from the gable end far enough to see both the eave and the ridge. Using a tool with an angle sensor, take an indirect measurement of the eave height and then the ridge height. Subtract one from the other to get the total rise of the roof.
Then measure the horizontal run, which is half the building width for a symmetrical gable roof, from the outside wall to the point directly below the ridge.
Pitch is then rise divided by run, expressed over 12. If the roof rises 5 feet across a 10 foot run, that is 0.5, which is 6/12.
This depends on the tilt sensor being accurate, so it is worth checking yours against a spirit level before relying on it. Tilt calibration is covered here, and the indirect measurement technique here.
Often the easiest, and completely safe.
Find an exposed rafter. Hold a level horizontally against the underside of the rafter, mark 12 inches along it from where it touches, then measure vertically from that 12 inch mark up to the rafter. That vertical distance in inches is your rise, and the pitch is that number over 12.
Measure two inches up and you have a 2/12 roof. Measure eight and it is 8/12. It takes a minute and needs nothing but a level and a rule.
Check two or three rafters in different parts of the roof. Older buildings settle, and a roof that reads 7/12 at one end and 6/12 at the other is telling you something useful about the structure.
A pitch gauge is a small dial tool that reads the angle directly when held against a rafter or a roof surface. They cost very little and are the fastest option if you do this regularly.
Phone inclinometer apps do the same job using the device's accelerometer, and are accurate enough for estimating provided you hold the phone flat against a straight edge rather than against the textured roof surface itself. Shingles are not a flat reference.
Only if you are properly equipped, and only if the ground methods have not given you what you need.
Hold a level horizontally on the roof surface, mark 12 inches, measure down to the surface. Same arithmetic as the attic method. It is the most direct reading and it is rarely worth the risk when three safer methods exist.
Two things follow immediately.
The pitch factor. Multiply your footprint area by this to get actual roof area. 4/12 is 1.054, 6/12 is 1.118, 8/12 is 1.202, 12/12 is 1.414. That last one means a very steep roof carries 41 percent more material than the ground it covers. The full table and worked examples are here.
What you are allowed to install. Low pitches restrict material choice, because water moves slowly and wind driven rain can travel back up under a lap. Manufacturers publish minimum pitches for their products and installing below them typically voids the warranty. Check the specific product rather than assuming, since the thresholds differ between manufacturers and between shingle types.
Very steep roofs bring their own consequences: more material, higher labour cost because the work is harder and slower, and in some cases additional fastening requirements.
Plenty of roofs are not a single plane. Dormers, additions, porches and mansards frequently differ from the main roof.
Measure each plane separately and calculate each one separately. Do not average them. A main roof at 8/12 and a porch at 3/12 averaged to 5.5/12 gives you a number that is wrong for both, and the porch may also be below the minimum pitch for the material you are planning to use on it.
Keep them as separate line items all the way through to the order. The full roof method is here.
If the question is really how to choose a laser measure rather than this specific tool, our front page is the better starting point.