How to Choose a Balance Quality Grade: G6.3, G2.5, and Everything Else

The work order says "balance to G2.5," and not another word after that: no rotor mass, no weight-mounting radius, no speed. Without these numbers, a grade doesn't turn into grams or into an acceptance criterion. Let's break down what's behind the letter G, how a grade turns into a g·mm tolerance, and why tightening the grade "just in case" costs more than it looks like.

Updated 27 August 2026 · by AXILINE · Vila Nova de Gaia

What Grade G Is, and What It Doesn't Regulate

The balance quality grade G is set by ISO 21940-11 (before revision, this part was called ISO 1940-1). The number in the grade designation equals the product of the allowable specific residual unbalance and the angular velocity, and it has units of mm/s. The notation G6.3 reads like this: e_per (the allowable specific residual unbalance) times ω (the angular velocity of rotation) equals 6.3 mm/s.

Specific residual unbalance is the total unbalance divided by the rotor's mass, in g·mm/kg. This quantity has an intuitive meaning: it's numerically equal to the displacement of the rotor's center of mass from the rotation axis, in micrometers. 40 g·mm/kg is 40 µm of eccentricity — four hundredths of a millimeter. A sheet of paper is about twice that thick.

Now, about the main source of confusion. The mm/s unit in the grade designation has nothing to do with the vibration velocity you read off a sensor at the bearing housing. G6.3 doesn't mean 6.3 mm/s on the housing, and zone A on the overall vibration-level scale doesn't confirm a G6.3 grade. These are two independent criteria with different units and different control points. We covered them together with a third one — the target residual 1x (vibration at the rotor's rotation frequency) in the instrument's software — in a separate article on the three tolerances in balancing.

The series of grades steps up by roughly a factor of 2.5 each time: G0.4, G1, G2.5, G6.3, G16, G40, G100, G250, G630, G1600, G4000. The upper grades address slow-turning, rigidly mounted systems; the lower ones address precision rotors. On-site work in practice lives almost entirely within the G16…G1 range.

Sources: ISO 21940-11:2016

Why a Grade Has Units of Speed

The standard's logic is simple, and it explains why you can't set a single eccentricity limit for every machine. The centrifugal force from an unbalanced mass grows in proportion to the square of the angular velocity. A rotor with its center of mass offset by 40 µm at 1500 rpm loads the bearings four times less than the same rotor at 3000 rpm. The same residual unbalance is harmless on a slow machine and destructive on a fast one.

That's why the standard regulates not the eccentricity itself, but the product of e_per and ω. Physically, this is the linear velocity of the center-of-mass point as it orbits the axis at a radius of e_per. Hence the mm/s in the grade designation.

You'll feel the practical consequence the moment you start calculating grams. One grade is one curve, not one number. Double the speed, and the allowable mass drops by half. A rotor that meets G6.3 at 750 rpm with 32 grams of residual must fit within 8 grams at 3000 rpm.

The table is calculated for grade G6.3 and a 100 kg rotor, with numbers rounded for convenience. It shows the relationship, not a substitute for the calculation: check the applicable part and edition of the standard, and the method for splitting the tolerance between planes, for your specific machine.

Sources: ISO 21940-11:2016

How to Get From a Grade to Grams: The Calculation Logic

For a grade to become an acceptance criterion, you need four numbers. Without any one of them, talk of G2.5 stays just talk.

1. 01

   Gather the input data

   Rotor mass M in kilograms (specifically the rotating part, not the whole machine), running speed n in rpm, grade G, and the actual weight-mounting radius r in millimeters for each correction plane (a rotor cross-section where correction weights are mounted). Take the mass from the nameplate or weigh it; measure the radius with a tape on site, don't estimate it by eye.

2. 02

   Convert speed to angular velocity

   ω = 2π·n/60 rad/s. At 1500 rpm that's about 157 rad/s, at 3000 about 314, at 1450 about 152. Easy to remember: divide the speed by 9.55.

3. 03

   Calculate the specific tolerance

   e_per = G/ω. For G6.3 at 157 rad/s that works out to 0.040 mm, or 40 µm, or 40 g·mm/kg. That's the allowable displacement of the rotor's center of mass from the rotation axis after balancing.

4. 04

   Move to the full tolerance

   U_per = e_per·M, or in a convenient form, U_per = 1000·G·M/ω in g·mm. For a 100 kg rotor at 1500 rpm and G6.3, that comes out to about 4,000 g·mm for the whole rotor.

5. 05

   Convert to grams at your own radius

   m = U_per/r. At a 250 mm radius that's 16 g; at a 500 mm radius, just 8 g. The farther from the axis you mount the weights, the fewer grams correspond to the same tolerance. That's exactly why you can't just guess at the radius.

6. 06

   Split the tolerance between planes

   For a symmetric between-bearings rotor, the total tolerance is usually split in half: 2,000 g·mm and about 8 g per plane. For an overhung or asymmetric rotor, the split is calculated based on the position of the center of mass and the distances to the bearings, and you can no longer just divide by two.

The numbers in the examples are rounded so the logic is easy to follow. The formulas apply to a rotor in the rigid state. Take the exact calculation, the grade-to-machine-type mapping, and the rule for splitting the tolerance between planes from the applicable edition of ISO 21940-11 and from the documentation for your specific rotor.

Sources: ISO 21940-11:2016

Guidelines: Which Grade for Which Machine

The standard gives recommendations by machine type, and on site they add up to a short list. The numbers in the last two columns are calculated for the same 100 kg rotor at 1500 rpm with a 250 mm correction radius, so you can see the cost of tightening the grade, in grams.

This table is a working guideline, not an excerpt from the standard. Before contractual acceptance, open the applicable edition of ISO 21940-11 and check the grade for your rotor type: the standard breaks things down in more detail, and separate recommendations exist for some machine types.

Sources: ISO 21940-11:2016

Five Questions That Decide the Grade Choice

The fifth question rules out more unrealistic requirements than the first four combined. Let's look at it separately, because this is exactly where the idea of "let's just ask for a tighter grade, just in case" falls apart.

• Is there a requirement in the machine's documentation, its nameplate, or the contract? If so, the choice is already made for you. Your job is different: verify that the requirement is physically achievable on this machine with your correction method, and say so before the work starts, not after.
• What kind of machine is it, and what system is it part of? A fan on a steel roof frame and a grinding-machine spindle can't share the same grade, even if their rotor masses happen to match. Look at the component's purpose and at who suffers from the vibration: the bearings, the foundation, product quality, or the people nearby.
• What's the running speed, and does the machine have just one operating condition? The same grade at 3000 rpm gives half the tolerance it does at 1500. If the machine runs on a VFD across a speed range, calculate the tolerance at the maximum running speed and record that in the report.
• Is the rotor rigid or flexible? ISO 21940-11 is written for rotors in the rigid state. If the running speed is close to or above the rotor's first critical speed (the speed at which the shaft starts to bow noticeably), the rotor deforms while running, unbalance redistributes along its length, and grade G stops describing the behavior. A case like this is assessed against different documents, and balanced using a modal approach — accounting for the shaft's bending mode shapes.
• What will you mount the weights with, and where? Radius, the number of available positions, the minimum washer or tack-weld mass, whether drilling is possible. If the calculated tolerance turns out smaller than your weight resolution, the grade is unrealistic, and no number of runs will fix that.

The number of correction planes has nothing to do with grade selection: that's a separate decision based on the L/D ratio (rotor length to diameter) and speed, and we have our own article on it. But the number of planes does directly affect how the tolerance is split.

Why You Shouldn't Ask for G1.0 "Just in Case"

The honest way to work is this. Set the grade that matches the machine — usually G6.3 for fans, pumps, and general-purpose electric motors. Reach it, look at the spectrum (the breakdown of vibration by frequency), and compare overall vibration against the 1x running-speed component. If 1x still dominates and that's not enough for the customer, tighten the grade deliberately: by then you already know how many runs that costs and whether your weight resolution can handle it. But if something other than 1x dominates the overall vibration, tightening the grade won't achieve anything at all, and that's where vibration diagnostics starts, not balancing. We've covered the cases where balancing doesn't help at all in a separate article.

How a Grade Turns Into a Number on the Instrument's Screen

In the Balanset-1A software, the tolerance on residual unbalance lives in the "Balancing tolerance" field and is expressed in g·mm. You can type it in by hand if the number has already been calculated for you, or open a separate calculation window and get the tolerance using the ISO 1940-1 method — that is, the currently applicable ISO 21940-11. You set the grade and the input data, and the software outputs g·mm.

The second field the whole calculation depends on is "Mass mount radius" for each plane. The software uses the radius to convert the initial and residual unbalance from the measured vibration and weights, and to check whether it's within tolerance. Get the radius wrong by 20%, and the tolerance in grams drifts by that same 20%.

After the verification run, the software shows the achieved residual unbalance right next to the tolerance you set. That pair of numbers is what goes into the report from the archive: the calculated U_per and the actual residual. The archive separately stores a second, independent tolerance in mm/s of vibration. Don't confuse them: the first field is about the rotor; the second is about how the machine vibrates.

• Enter the actual mass of the rotating part, not the mass of the whole machine including the motor and frame.
• Measure and enter the real weight-mounting radius for each plane before you calculate the tolerance.
• Set the speed the machine actually runs at, and for a variable-speed drive, the maximum running speed.
• Check how the total tolerance is split across planes, especially for an overhung rotor.
• Enter the trial weight's mass in grams, not as a percentage, if you plan to balance this machine later using saved influence coefficients.
• Save the archive: it gives you a report with both tolerances and a polar diagram, not just a "within tolerance" figure.

Sources: ISO 21940-11:2016 · Balanset-1A operation manual

What to Put in the Contract and the Report

The phrase "balance to G2.5" isn't a technical requirement. It can't be verified and can't be disputed. For a grade to become an acceptance criterion, the contract needs all the numbers that turn it into grams, plus a method of verification.

• Grade G and the standard, with the part and edition year stated.
• The rotor mass used in the calculation, and exactly what it includes: just the wheel, the wheel with the shaft, or the assembled rotor.
• The running speed the tolerance is calculated for, and behavior with a variable-speed drive.
• The actual mounting radius for correction weights in each correction plane.
• The calculated allowable residual unbalance in g·mm, and the rule for splitting it between planes.
• The method of verification: calculating residual unbalance from residual 1x and influence coefficients in situ, in the machine's own bearing housings. This isn't the same as checking the rotor on a balancing machine, and one shouldn't be substituted for the other.
• As a separate line, the overall-vibration criterion referencing the applicable part of ISO 20816, with measurement points and directions, frequency band, and machine condition. This is a second, independent requirement, and it doesn't follow from grade G.
• What each party does if the grade is achieved but overall vibration is still above the limit. This is usually a separate stage: diagnostics and mechanical repair, shaft alignment, fasteners, supports.

There's no direct conversion from grade G to mm/s at the bearing housing. The same residual unbalance on a massive foundation and on a flexible steel frame will produce different housing vibration, because that vibration also depends on the mass and stiffness of the supports, damping, and closeness to resonance. That's why the two criteria are checked separately and both go into the report.

Sources: ISO 21940-11:2016 · ISO 20816-1:2016

How AXILINE Can Help

Choosing the grade is half the job, and usually the most contentious half. We do it together with the customer before the site visit: we look at the machine type, speed, rotor mass, and where a weight can physically be mounted, calculate the tolerance in g·mm and in grams, and say plainly whether it's achievable on this machine or not. If a requirement from the spec is unrealistic given the correction resolution, you find out in advance, not at the end of the shift.

From there we come to the machine, mount sensors on the bearing housings, separate the running-speed component from the overall level, and look at the spectrum. If unbalance really does dominate, we balance the rotor in its own bearing housings, in one or two planes, and hand over a report where the calculated tolerance and the achieved residual unbalance sit side by side, with the overall-vibration criterion on its own line. If unbalance isn't the main cause, you get vibration diagnostics telling you what to fix before balancing.

The work is carried out by engineers who design and manufacture Balanset instruments and do their own on-site balancing with them. If you want to calculate grade-G tolerances and keep your own reports, the Balanset-1A can be bought: two vibration sensors, a laser phase sensor, a two-channel USB module, and Windows software with tolerance calculation, an archive, and a polar diagram. Consulting support stays with us either way.

Tell us the machine type, speed, rotor mass, and the grade you're being asked for. That's usually enough for us to tell you how many grams of residual you're allowed and whether hitting that is realistic.

Sources: Balanset-1A manufacturer specification

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