Quick Answer

Use a 60° gloss meter as the universal reading angle on polished concrete; switch to 20° for high-gloss mirror finishes above 70 GU, and to 85° for matte or low-sheen areas below 10 GU. Clean the slab, calibrate the meter on the black glass tile every session, and take readings on a grid every 3 to 6 meters across the floor. Typical targets are 20 to 30 GU for satin, 40 to 60 GU for semi-polished, and 70 to 90 GU for high-gloss mirror finish, measured at 60°. Always compare readings on the same slab under the same lighting, because GU is a relative number, not a visual one.

Why Measurement Angle Matters

A gloss meter shoots a beam of light at the surface and measures the light reflected at the mirror angle. Different angles read different surfaces: a shallow 20° beam is reflected strongly by high-gloss surfaces but poorly by matte ones, while a steep 85° beam is sensitive to texture on low-sheen floors but saturates on mirrors. Using the wrong angle gives numbers that look impressive but say nothing about the floor. A high-gloss mirror read at 85° will peg near the top of the scale and hide variation; a matte floor read at 20° will read near zero and hide a dull spot. The angle is chosen by the sheen range, not by habit.

Readings are only comparable when the angle is the same. Two contractors can measure the same floor and report 80 GU and 20 GU without disagreeing, because one used 60° and the other 20°. On a bid or a punch list, always record the angle next to the GU number. A floor that reads 75 GU at 60° is a high-gloss job; a floor that reads 75 GU at 85° is a semi-gloss floor.

The Three Standard Reading Angles

20° for high-gloss mirror finishes

The 20° geometry is the high-sheen angle. It discriminates best above 70 GU at 60°, which is where high-gloss polished concrete, 3000-grit resin burnished floors, and sealed mirrors sit. Use 20° once the floor reads above 70 GU at 60°, because that is where 60° starts to compress differences. A high-gloss floor that reads 75 GU at 60° may read 90 GU at 20° in one bay and 70 GU at 20° in another, exposing variation that 60° hid. The 20° meter has a small aperture, so hold it flat and read only on a clean, dry spot.

60° for universal mid-sheen reading

60° is the default angle for polished concrete because it covers the widest range of finishes, from satin to semi-gloss to high-gloss. Use 60° for every floor until a reading pushes above 70 GU or drops below 10 GU, then switch to the matching angle. Most polished concrete jobs sit in the 20 to 80 GU at 60° range, and 60° is what clients, specifiers, and punch lists expect. Keep 60° as the baseline reading across the floor, and use 20° or 85° only where the baseline falls outside its comfortable window.

85° for matte and low-sheen areas

The 85° geometry is the low-sheen angle. It is sensitive to texture and reads variation on floors below 10 GU at 60°, which includes matte concrete, unsealed burnished floors, and the dull edge band beside a wall. Use 85° when the 60° reading drops below 10 GU and you need to tell a dull patch from a normal matte finish. A floor that reads 8 GU at 60° may read 30 GU at 85° in a good bay and 15 GU at 85° in a dull spot, which is the difference that raking light will show the client.

Typical GU Target Ranges

Typical polished concrete finishes read as follows at 60°: 200-grit satin around 10 to 25 GU, 400-grit semi-polished around 30 to 50 GU, 800-grit polished around 50 to 70 GU, and 1500 to 3000-grit high-gloss around 70 to 90 GU. These are typical ranges, not guarantees, because slab hardness, densifier, and sealer change the final number. A soft slab never reaches 80 GU at 60° no matter how fine the grit; a dense slab can overshoot and needs a matte sealer to bring the sheen down.

Meter Setup and Calibration

Calibrate the meter on its black glass calibration tile at the start of every session, and re-calibrate every two hours or whenever the meter moves between cool and warm environments. Low batteries and temperature swings are the two reasons a meter drifts mid-shift. Clean the reading aperture with a microfiber cloth before each session, because a fleck of dust under the sensor changes the reading by several GU. Do not read over a joint, a scratch, or a wet spot, because the aperture is small and any defect inside it skews the number. Move the meter a few centimeters until the reading stabilizes on uniform concrete.

• Clean the slab surface with a dry microfiber mop; no dust, no sealer residue, no water.
• Calibrate the meter on the black glass tile at the start of each session and every two hours.
• Read on a 3 to 6 meter grid across the floor, skipping joints and expansion gaps.
• Use 60° as the baseline; switch to 20° above 70 GU or 85° below 10 GU.
• Record each reading with the angle, the grid location, and the time on a floor plan.

Reading Grid and Consistency

Take readings on a regular grid, roughly every 3 to 6 meters, so you can compare bays and spot low spots. Read the same spots at the same time of day, because sunlight through a window changes ambient light and the meter reading. A floor that reads 60 GU in the morning can read 65 GU in afternoon sun, and that swing is lighting, not the floor. For punch-list comparisons, close the blinds, use the room's artificial lights, and read the same spots in the same order.

Comparing bays on the same slab

Compare readings on the same slab, not across slabs. GU numbers are relative to the surface and the sealer, so a 70-GU read on a dense warehouse floor is not the same finish as a 70-GU read on a residential slab. The useful comparison is between bays on the same job: if the front bay reads 65 GU and the back bay reads 45 GU, the back bay needs another fine pass or another coat of sealer before the client accepts it.

FAQ

Which angle should I buy on a gloss meter? Buy a 20°/60°/85° multi-angle meter. The 60° handles most polished concrete, the 20° covers high-gloss mirror floors, and the 85° covers matte or low-sheen work. A single-angle 60° meter is a budget start but will not tell you enough on high-gloss jobs.

How often should I calibrate the meter? Calibrate on the black glass tile at the start of every session and re-calibrate every two hours, after any temperature swing, and before taking readings for a client punch list. A drifted meter gives consistent-looking but wrong numbers, which is worse than no meter at all.