Scale Accuracy and Tolerance: How to Check, Calculate and Keep a Scale Accurate

Updated November 9, 2026

The short answer: scale accuracy is how close a scale's reading is to the true weight of the load. It depends on calibration, linearity, repeatability and the conditions the scale works in, not just on how fine the display reads. For commercial scales in the US, "accurate enough" has a number: the tolerance in NIST Handbook 44, worked out from the scale's accuracy class, its division size and the test load. Below: what affects accuracy, how to calculate your scale's tolerance with worked examples, how to check a scale yourself, and how to keep it accurate.

What scale accuracy means

Put a known 100 lb test weight on a scale. If it reads 100 lb, the scale is accurate at that load. If it reads 101 lb, the error is 1 lb, and whether that is acceptable depends on the scale. A 1 lb error is fine on a 10,000 lb floor scale with 2 lb divisions and far too much on a 30 lb bench scale.

That is why accuracy is always judged against two things: the scale's division size (the step the display moves in, written as d) and the tolerance allowed for its accuracy class. A scale is "accurate" when its errors stay inside that tolerance across its whole range, every time you weigh.

Readability, linearity, repeatability and corner balance

Readability (division size)

Readability is the smallest change the display shows, for example 1 lb, 0.1 lb or 0.01 g. It sets how finely you can read the result, but it is not accuracy. A scale that reads to 0.1 lb can still be 2 lb off if it is out of calibration. Choose readability for the job: livestock and pallet weighing rarely needs better than 1 to 2 lb, while lab work may need 0.1 mg.

Linearity

Linearity is how well a scale stays accurate from zero to full capacity. A scale can be spot-on at 100 lb and drift at 1,000 lb if its response isn't a straight line. That is why a proper check uses several test loads across the range, not one weight near the bottom.

Repeatability

Repeatability is whether the same load gives the same reading time after time. Weigh the same item five times, removing it in between. If the readings wander, look for vibration, air currents, a load touching something, or a failing load cell before you blame calibration.

Corner balance (trim)

On floor, livestock and truck scales, the reading should be the same wherever the load sits on the platform. If one corner reads high or low, the scale is out of trim. On multi-load-cell scales this is set in the junction box by a qualified scale technician.

Scale tolerance: how to calculate it

Tolerance is the amount of error allowed at a given test load. US commercial (legal-for-trade) scales follow NIST Handbook 44, which sets tolerances in divisions by accuracy class:

• Class II: precision laboratory balances and other high-resolution scales.
• Class III: most commercial scales, including bench, counter and floor scales.
• Class III L: vehicle, axle-load, livestock, railway and crane scales.

Handbook 44 Table 6 gives the maintenance tolerance (for scales in use). Acceptance tolerances, which apply to new or newly repaired scales, are half of these values.

For Class III loads above 4,000 d, and for Class I and IIII, use Table 6 in the current edition of Handbook 44. Your state may adopt a particular edition, so confirm with your state weights and measures office.

How to calculate scale tolerance, step by step

1. Find the division size (d) on the scale's data plate or indicator settings, for example 1 lb.
2. Convert the test load to divisions: test load ÷ d. A 1,000 lb test load on a 1 lb scale is 1,000 d.
3. Find the band for your class in the table. For Class III, 1,000 d falls in 501 to 2,000 d, so the maintenance tolerance is 2 d.
4. Convert back to weight: 2 d × 1 lb = ±2 lb. The scale passes at that load if it reads between 998 and 1,002 lb.

Worked examples

• Bench scale, Class III, 0.01 lb divisions, 20 lb test load: 20 ÷ 0.01 = 2,000 d, so 2 d = ±0.02 lb maintenance, ±0.01 lb acceptance.
• Floor scale, Class III, 1 lb divisions, 2,500 lb test load: 2,500 d falls in 2,001 to 4,000 d, so 3 d = ±3 lb maintenance, ±1.5 lb acceptance.
• Livestock or truck scale, Class III L, 20 lb divisions, 20,000 lb test load: 20,000 ÷ 20 = 1,000 d, so 2 d = ±40 lb maintenance, ±20 lb acceptance.

Two more rules from Handbook 44 matter in practice. Repeated weighings of the same load should agree within the maintenance tolerance. And the test weights themselves should have an error no more than one-third of the smallest tolerance you are checking, which is why the calibration weight class you buy matters.

These numbers are general guidance. If you weigh for trade, a qualified scale technician or your state weights and measures office decides whether a scale passes. Our NTEP and legal-for-trade guide covers what that involves.

How to check if a scale is accurate

You can't certify a scale yourself, but a quick check between calibrations catches problems early:

1. Warm up and zero. Turn the scale on, let it settle, and confirm it returns to zero with nothing on it.
2. Weigh a known test weight near the middle of your normal working range, then one near the heaviest loads you weigh. Compare each reading with the tolerance you worked out above.
3. Repeat it. Weigh the same weight three to five times. The readings should agree.
4. Check the corners on platform scales: place the weight in each corner and the center. The readings should match.
5. Log the results with the date, so you can spot drift over time.

Our test weights guide explains how much test weight you need for bench and floor scales, and the bench scale calibration guide walks through calibrating a Liberty bench scale. If a scale fails a check, stop using it for trade and call a qualified scale technician.

For a deeper look at how calibration is done in the field, see Scale Blog's industrial scale calibration guide.

What throws a scale off

• Uneven or flexing floors. A platform scale needs a flat, solid, level base. A rocking corner changes the reading.
• Debris and binding. Dirt, ice, manure or a pallet edge touching the frame or a wall takes weight off the load cells.
• Overloads and shock loads. Dropping loads or going over capacity can permanently damage load cells. Never load a scale beyond its rated capacity.
• Temperature and moisture. Big temperature swings and water in cables or junction boxes cause drift. Let the scale reach room temperature before weighing.
• Vibration and air currents. Nearby machinery, fans and open doors make sensitive scales and balances unstable.
• Damaged cables. A pinched or chewed load cell cable gives jumping or drifting readings.

How to keep a scale accurate

• Calibrate on a schedule. Most businesses calibrate at least once a year, and more often for heavy use, harsh conditions or legal-for-trade weighing. Calibrate again after a move, a repair or an overload.
• Check between calibrations with a known test weight, as above.
• Keep it clean and dry, especially around the edges of the platform and under it.
• Load it properly: place loads gently and in the center, and stay within capacity.
• Protect cables and junction boxes from traffic, animals and water.
• Use the right scale for the job. For lab weighing, our analytical vs precision balance guide explains which readability you actually need.

Test weights for accuracy checks

To check a scale, you need weights you can trust. Liberty's grip handle test weights cover most bench and floor scale checks (prices from libertyscales.com on November 9, 2026):

See all calibration and test weights. Not sure what you need? Call Liberty Scales at (661) 888-1919 or send us a message, and tell us the scale's capacity, division size and what you weigh.

Scale accuracy FAQs

What is the difference between scale accuracy and readability?

Readability is the smallest step the display shows, such as 1 lb or 0.01 g. Accuracy is how close the reading is to the true weight. A scale can show fine steps and still read wrong if it is out of calibration, so a fine readability is not proof of accuracy.

How do I calculate the tolerance for my scale?

Divide the test load by the scale division to get the load in divisions, find that band for your accuracy class in NIST Handbook 44 Table 6, and multiply the tolerance in divisions by the division size. For a Class III scale with 1 lb divisions, a 1,000 lb test load is 1,000 d, which has a maintenance tolerance of 2 d, or plus or minus 2 lb.

What is the difference between acceptance and maintenance tolerance?

Acceptance tolerances apply to a new or newly repaired scale and are half of the maintenance tolerances. Maintenance tolerances apply to scales already in use. Your state weights and measures office decides which one applies to a given inspection.

How often should a scale be calibrated?

Most businesses calibrate at least once a year, and more often for heavy use, harsh conditions or legal-for-trade weighing. Check the scale yourself with a known test weight between calibrations, and after any move, overload or repair.

Why does my scale give a different weight when I move the load?

If the reading changes when the same load sits in a different spot, the scale is out of corner balance (trim). On multi-load-cell scales a qualified scale technician adjusts the trim in the junction box. Damaged load cells, debris under the platform or an uneven floor can cause the same symptom.