Unilateral tolerance is a dimensioning method in Intro to Engineering that lets a part vary only above or below the nominal size, not both. It shows a one-sided acceptable range on a drawing.
Unilateral tolerance is a one-sided size limit on an engineering drawing. In Intro to Engineering, you use it when a dimension can move only in one direction from the nominal size, either only larger or only smaller, depending on what the part needs to do.
The nominal size is the target dimension, and the tolerance tells you how far the actual part can vary. With unilateral tolerance, the drawing might allow something like 10.00 +0.20/0.00, which means the part can be 10.00 mm or slightly bigger, but not smaller than 10.00 mm. A lower unilateral version would do the opposite, allowing only smaller values.
This differs from a bilateral tolerance, where variation is allowed on both sides of the nominal size. Unilateral tolerance is more controlling because it protects one important boundary. If a shaft must never get too large, or a hole must never get too small, a one-sided limit gives the manufacturer a clear rule.
You will usually see unilateral tolerance on dimensioned drawings when fit or function depends on a single direction of error. For example, if a bracket slot has to accept a mating part, the designer may allow the slot to grow a little but not shrink below the minimum opening. That keeps the part usable without making the tolerance more complicated than needed.
In manufacturing, this choice affects inspection too. A machinist, CAD user, or quality checker reads the tolerance and decides whether a measured part passes or fails. If the part crosses the allowed side, it is out of spec even if the dimension is close to the nominal value.
A common mistake is treating unilateral tolerance like a general plus or minus symbol. In engineering drawings, the direction matters. The real question is not just how much variation is allowed, but which side of the nominal size is safe for the design.
Unilateral tolerance shows how engineering drawings translate a design idea into a buildable part. Intro to Engineering often focuses on communication, and this is a good example of why drawings cannot be vague. A dimension that is off by the wrong amount can stop a part from fitting, moving, or sealing correctly.
This term connects directly to part function. If a hole is supposed to accept a bolt, the opening may need a minimum size. If a peg needs to stay inside a slot without wobbling, the designer may care more about preventing the peg from getting too big. A unilateral tolerance lets the drawing protect that one critical limit.
It also ties into manufacturing tradeoffs. Tighter control on only one side can simplify production compared with balancing two-sided variation. That matters when you are choosing tolerances in a class project, a CAD assignment, or a design review, because overly strict limits can raise cost or make the part harder to produce.
You also need this term to read drawings correctly. In Intro to Engineering, a lot of mistakes happen when someone sees a dimension and assumes any nearby value is fine. With unilateral tolerance, one direction is acceptable and the other is not, so the sign and the side of the limit change how you inspect the part.
Keep studying Intro to Engineering Unit 7
Visual cheatsheet
view gallerynominal size
Nominal size is the target dimension that unilateral tolerance is built around. The tolerance does not replace the nominal value, it tells you how far the real part may move from it. When you read a drawing, start with the nominal size, then check which side of that size is allowed to vary.
bilateral tolerance
Bilateral tolerance allows variation on both sides of the nominal size, while unilateral tolerance only allows one side. That difference changes how strict the part is and how you interpret a drawing. If the function only cares about one boundary, unilateral tolerance is often the cleaner choice.
Clearance fit
Clearance fit is about making sure two parts can assemble with enough space between them. Unilateral tolerance often shows up in drawings for clearance fits because one side of the dimension may need to stay above a minimum opening. The tolerance helps keep the fit usable without guessing.
Functional dimensioning
Functional dimensioning focuses on the measurements that affect how a part works, not just how it looks on paper. Unilateral tolerance fits that idea because it protects the dimension that matters most for function. In a project, you use both to prioritize the sizes that affect performance.
A quiz or drawing-reading question may show a dimension with a one-sided tolerance and ask you to identify the acceptable size range. You might also need to decide whether a part passes inspection after measuring it, so you check the nominal size and the allowed direction before answering. In a CAD or drafting task, you may be asked to add the correct tolerance to a feature that must stay within one limit. If the question mentions fit, assembly, or a minimum opening, unilateral tolerance is usually the clue that one boundary matters more than the other.
These are easy to mix up because both describe allowable variation around a nominal dimension. Bilateral tolerance allows error in both directions, but unilateral tolerance only allows error on one side. If a problem says the part can be larger or smaller within equal limits, that is bilateral. If it protects only one maximum or minimum, that is unilateral.
Unilateral tolerance lets a dimension vary in only one direction from the nominal size.
The drawing will show which side is allowed, so the sign and limit direction matter.
Engineers use it when one boundary is more important than the other for fit or function.
It makes inspection straightforward because a part either stays inside the one-sided limit or it does not.
If both sides of the nominal size are allowed, you are looking at bilateral tolerance instead.
Unilateral tolerance is a dimensioning rule that lets a part vary only above or only below the nominal size. It appears on engineering drawings when one limit matters more than the other, such as protecting a minimum opening or a maximum shaft size. That makes the design easier to inspect and more precise for the part’s job.
Look at the nominal dimension first, then check the one-sided deviation shown beside it. The acceptable values all stay on one side of the target size, not both. If the tolerance is written as +0.20/0.00, for example, the part can be at the nominal size or up to 0.20 larger, but not smaller.
Bilateral tolerance allows variation above and below the nominal size, while unilateral tolerance allows variation in only one direction. That means bilateral tolerance is centered around the target, but unilateral tolerance protects a single limit. Designers choose unilateral when one boundary is critical for assembly or function.
An engineer uses unilateral tolerance when only one side of the dimension can safely change. That might happen if a hole cannot get too small, a shaft cannot get too large, or a slot has to stay open enough for assembly. It gives tighter control where the part needs it most without over-restricting the other side.