ISO IT Tolerance Grades Explained — IT01 Through IT18 and What Grade a Forged or Machined Feature Actually Needs
A practical explainer on the ISO International Tolerance (IT) grade system — IT01 through IT18 — used to specify dimensional tolerance on drawings, and how it applies differently to as-forged surfaces versus subsequently machined features. Shivam Forge, Rajkot, India. Call +91-9265772827.
The ISO International Tolerance (IT) grade system, defined in ISO 286, provides a standardized way to specify how much a dimension is permitted to deviate from its nominal value, expressed as a grade number rather than a raw tolerance value — IT01, IT0, and IT1 through IT18, running from extremely tight to comparatively loose. The genuinely useful property of this system is that the actual permitted tolerance in millimeters or microns for a given IT grade scales with the nominal dimension size, since a tolerance that's appropriately tight for a 10mm feature would be needlessly restrictive, or conversely inadequate, if applied unchanged to a 500mm feature — the IT grade abstracts this relationship into a single number that a designer, machinist, or forge shop can reference against a standard table for the actual permitted deviation at any given size. For forged components specifically, understanding where IT grades apply matters because a single forging typically combines surfaces at genuinely different tolerance grades: as-forged surfaces, which inherit the natural process variation of hot metal forming into a die cavity, sit at the loose end of the practical range (commonly in the IT14–IT16 territory, sometimes described instead through forging-specific tolerance standards rather than IT grades directly), while subsequently machined features on that same forging — a bearing bore, a shaft journal, a precision mating face — can be brought to much tighter IT grades (IT6–IT9 is common for functional machined fits) through secondary machining operations. Recognizing which features on a drawing genuinely need a tight IT grade, and which can remain at looser as-forged or coarser machined tolerance, is central to both accurate quotation and sensible engineering, since specifying unnecessarily tight tolerance across an entire drawing inflates cost without functional benefit.
An IT grade (IT7, for example) doesn't specify a single tolerance value directly — it specifies a tolerance band width that's looked up against the feature's nominal dimension in the ISO 286 standard table, meaning the same IT7 grade represents a tighter absolute tolerance in microns on a small-diameter feature than on a large one, while representing a comparable relative precision across the whole size range.
The complete IT grade scale runs from IT01 and IT0 (extremely tight, essentially gauge-block-level precision, rarely relevant to forged component features) through IT18 (very loose, closer to rough casting or basic structural tolerance). Practical forged and machined component work concentrates in a much narrower functional band within this full range.
Surfaces left in the as-forged condition — not subsequently machined — carry the inherent dimensional variation of the hot forming process itself, and typically fall in the IT14–IT16 range, or are alternatively specified through forging-specific tolerance standards (such as those referenced in forging process standards) that serve a similar function to IT grades but are calibrated specifically to forging process capability rather than machining capability.
Once a feature is machined — a bore, a shaft diameter, a flat mating face — dramatically tighter tolerance becomes achievable, with IT6 through IT9 covering the range typical for functional fits (bearing bores, shaft journals, gear seats), and even tighter grades reserved for genuinely precision applications like gauge or reference surfaces.
A drawing that applies a uniformly tight IT grade across every dimension — including features with no genuine functional precision requirement — forces a supplier to quote conservatively against the drawing as written, inflating cost without corresponding functional benefit. Reserving tight IT grades specifically for dimensions that drive fit, function, or assembly, and leaving other dimensions at a looser, more economical grade, is the more cost-effective approach.
Clearly indicating which surfaces are left as-forged (and therefore governed by forging process tolerance capability) versus which are machined to a specific IT grade avoids ambiguity that can otherwise lead a supplier to either over-machine unnecessarily or under-deliver on a genuinely functional tolerance requirement.
Tighter IT grades generally require correspondingly more capable and controlled machining processes, additional inspection, and in some cases secondary finishing operations (grinding, honing) beyond standard turning or milling — meaning the IT grade specified doesn't just describe an outcome, it effectively drives which manufacturing process and inspection regime is required to reliably achieve it.
State the specific IT grade or numeric tolerance value for critical dimensions explicitly, rather than relying on a general title-block default tolerance applied uniformly — a supplier reading a drawing with clearly flagged critical-dimension IT grades can quote accurately and plan the correct process on the first pass, rather than assuming worst-case tolerance across the entire part.
The ISO International Tolerance grade system, formally defined in ISO 286, exists to solve a specific practical problem in dimensional specification: describing how much a feature's actual size is permitted to deviate from its nominal, intended value, in a way that scales sensibly across the enormous range of feature sizes real components span. Rather than specifying a fixed tolerance value that would be either needlessly restrictive on large features or inadequately loose on small ones, the IT grade system defines a series of numbered grades — IT01, IT0, and IT1 through IT18 — where each grade represents a consistent relative precision level, with the actual permitted tolerance in millimeters or microns for any given grade looked up against the feature's nominal dimension in the standard's reference tables. This size-scaling relationship is what makes the IT grade system genuinely useful as a shared reference language between designers, machinists, and forge shops working across a huge range of component sizes.
For forged components specifically, understanding where IT grades apply — and where they don't quite apply in their machining-calibrated form — matters because a single forging typically spans genuinely different tolerance regimes across its different surfaces. As-forged surfaces, left exactly as they emerge from the die without subsequent machining, carry the inherent dimensional variation of hot metal forming: die wear, thermal contraction during cooling, and the practical limits of controlling material flow within a die cavity all contribute variation that's simply a different order of magnitude from what machining can achieve, commonly falling in the IT14–IT16 range when expressed in IT grade terms, or alternatively specified through forging-specific dimensional tolerance standards developed specifically around forging process capability rather than borrowed from machining tolerance conventions. Machined features on that same forging — a bearing bore turned to final size, a shaft journal ground to a precision fit, a flat face milled for a gasketed joint — operate in an entirely different tolerance regime, with IT6 through IT9 covering the range typical of genuinely functional fits, and even tighter grades available where specific applications (precision gauging, certain bearing arrangements) demand it.
The practical consequence for anyone specifying or quoting a forged component drawing is that IT grade selection should track actual function, not drawing convenience. A dimension that has no genuine role in fit, assembly, or mechanical function gains nothing from a tight IT grade beyond inflated manufacturing cost and inspection burden — the tighter grade simply forces a more controlled, more heavily inspected process onto a feature that didn't need it. Conversely, a dimension that genuinely does drive a critical fit — a bearing bore diameter, a shaft-to-hub interface, a precision mating flange — needs its IT grade specified explicitly and clearly distinguished from the drawing's general default tolerance, since leaving a functionally critical dimension under a loose blanket tolerance risks a part that doesn't actually perform as intended even though it technically conforms to the drawing as literally written. Getting this distinction right on a drawing — explicit IT grades or numeric tolerances on truly critical features, sensible looser tolerance or as-forged condition elsewhere — is one of the most direct ways a buyer can influence both quoted cost and delivered part quality.
For engineers and buyers working through tolerance specification on a forged component drawing, or wanting to confirm whether a specified IT grade is achievable as-forged, requires secondary machining, or could reasonably be relaxed without functional impact, Shivam Forge's engineering team reviews this as a standard part of every quotation. Contact us at +91-9265772827 or sales@shivamforge.com with your drawing and tolerance requirements for a manufacturability review.
IT7 doesn't correspond to a single millimeter value on its own — it's a grade that's looked up in the ISO 286 standard table against the feature's actual nominal dimension, since the permitted tolerance band scales with size. A small-diameter feature at IT7 will have a tighter absolute tolerance in microns than a large-diameter feature at the same IT7 grade, even though both represent a comparable relative precision level.
As-forged surfaces are sometimes described using IT grades in the IT14–IT16 range, but are frequently instead specified through forging-specific dimensional tolerance standards calibrated directly to hot forging process capability rather than machining capability — either approach is common, and the important point for a buyer is understanding that as-forged tolerance is inherently looser than what machining can achieve, and specifying accordingly.
A tight IT grade drives which manufacturing process and inspection regime is required to reliably achieve it — applying a tight grade to a dimension with no genuine functional precision requirement forces a supplier to plan additional machining passes, tighter process control, or extra inspection for a tolerance that doesn't actually matter to the part's function, inflating quoted cost without functional benefit.
This depends on the specific bearing type, fit class, and application, but functional rotating fits commonly fall in the IT6–IT8 range — your bearing or component supplier's fit recommendation, combined with the specific ISO fit designation (such as H7 or k6) referencing the relevant IT grade, is the right basis for a precise specification rather than a general rule of thumb alone.
Yes. Our engineering team reviews incoming drawings and can flag where a specified IT grade appears unnecessarily tight relative to the feature's apparent function, or confirm the correct process and inspection approach needed to reliably achieve a genuinely critical tolerance requirement.
Why Choose Shivam Forge
Shivam Forge delivers precision hot-forged components from our integrated Shapar, Rajkot facility — covering forging, CNC machining, heat treatment, and quality inspection under one roof.
