When a supplier quotes a 12-inch log, the log that lands on your pad will measure something less. Milled logs for log home construction are sold by nominal diameter, and the actual dimension depends on how the log was milled, how much moisture it lost in drying, and which species it came from. The same naming gap runs through the rest of construction. A concrete stair carries nominal reinforcement for a 1-meter-high flight on paper, lumber is sold by nominal sizes, and builders who never check the actual numbers can end up with members smaller than the drawings assume.
This article explains where the difference comes from, how much shrinkage you can reasonably expect, and which questions to put to a supplier before you sign. The goal is to order a package knowing what the finished diameter will be, so heights, clearances, and insulation details still work when the walls go up.
What Nominal Means When You Order a Log Package
Nominal describes the label; actual describes the material. For milled logs, the nominal size refers to the rough-cut diameter before the log is milled and dried, and the actual dimension is the size after both processes are complete. Every log package carries that distinction, though not every sales sheet states it clearly.
Structural engineers work with the same vocabulary. When they check nominal reinforcement requirements for staircases on solid concrete supports, they compare the bar size written on the drawing with the strength of the real section, because the label alone does not guarantee performance. Log buyers should read contracts the same way: the nominal diameter on the order form is a starting point, not a promise about the finished wall.
Nominal versus actual in one sentence
The nominal size is what the log is called; the actual size is what the log measures once milling and drying are done. Most manufacturers quote nominal diameters in whole inches, such as 8, 10, or 12, because those are the numbers used in marketing, pricing, and plan review.
Where the confusion starts
The confusion starts when a buyer assumes the label is the measurement. A 12-inch nominal log is advertised as a 12-inch log, yet the finished diameter can be half an inch or more smaller after the lathe and the kiln do their work. Window jambs, door frames, and electrical chases planned around the nominal number run tight once the real diameter shows up on site.
Why Milling and Drying Shrink the Diameter
Two processes move a log from rough-cut to finished: milling and drying. Each one removes material, and together they explain most of the gap between nominal and actual.
Milling: cutting to a round profile
A rough log is not round. Milling spins the log against lathe blades or passes it through a profile machine that cuts a consistent circular cross-section. The machine strips bark, taper, and unevenness, so the starting size drops the moment the cut begins. Manufacturers can mill to a specific diameter, which is why nominal sizes come in round numbers.
Drying: moisture leaves, dimensions follow
Wood shrinks as it dries, and a log loses measurable diameter from green to dry. The amount depends on the species, the starting moisture content, and how the log is sawn. Kiln-dried logs reach lower moisture content faster than air-dried ones, and the total movement shows up in the final measurement.
Green versus kiln-dried shrinkage
A green log cut at high moisture content can lose more diameter in drying than a partially seasoned one. Most of the movement happens as the log passes below the fiber saturation point, roughly 28 to 30 percent moisture content. Below that threshold the cell walls begin to shrink and the cross-section follows.
Tolerances you can reasonably expect
Dimensional lumber has published standard tolerances, but those standards describe linear dimensions of boards, not the diameter of milled logs. In practice, a 1/16-inch or 1/4-inch variation may be acceptable, depending on the product and the supplier. Concrete has a parallel vocabulary problem: a nominal mix and a design mix of concrete are different things, the first a recipe and the second a tested proportion, and confusing the two changes the strength of the slab. The lesson transfers directly to logs: ask what the label actually guarantees.
The Lumber Analogy: 2x4s and the 1×12 Example
Anyone who has bought lumber knows this game. A nominal 1-by-12 board measures 3/4 inch thick and 11-1/4 inches wide after milling and drying. A 2×4 measures 1-1/2 by 3-1/2 inches. The reductions are consistent for boards because grading rules standardize them, which is why nominal vs actual lumber dimensions matter before you cut: a plan written for the nominal size produces joints off by a quarter inch or more.
The board example is not a direct equivalent to a log diameter, but it shows the same reduction from nominal to actual. The difference is that lumber reductions are standardized and predictable, while log reductions vary by milling practice, species, and moisture content.
What the 1-by-12 example teaches
Three lessons carry over from boards to logs:
- The label is a pre-processing size, not a delivered size.
- The reduction is real, repeatable, and permanent.
- The supplier specifications are the only reliable source for the final number.
With boards, those specifications are public grading rules. With logs, they are manufacturer data, so you have to ask.
| Nominal size | Actual size (inches) | Typical use |
|---|---|---|
| 1×4 | 3/4 x 3-1/2 | Trim, furring |
| 1×6 | 3/4 x 5-1/2 | Siding, shelving |
| 1×12 | 3/4 x 11-1/4 | Shelving, benchtops |
| 2×4 | 1-1/2 x 3-1/2 | Wall framing |
| 2×6 | 1-1/2 x 5-1/2 | Framing, joists |
| 2×10 | 1-1/2 x 9-1/4 | Joists, headers |
Logs have no equivalent published table, which is exactly why the nominal-to-actual gap for a log package has to be confirmed with the mill.
Species, Moisture, and Sawing Method Change the Outcome
The same nominal log from two different mills can finish at two different diameters, and a single mill can produce different results from different species. The reduction depends on the specific milling practices, the wood species, and the moisture content at delivery.
How species shifts shrinkage
Denser species generally move more as they dry. A softwood log and a hardwood log cut to the same nominal diameter can lose different amounts of diameter, and even two softwoods, such as pine and spruce, shrink at different rates. The mill data for the exact species you are buying is the number that matters.
Flat-sawn versus quartersawn cuts
How a log is cut also changes movement. Flat-sawn and quartersawn pieces shrink differently across their width, and the sawing pattern affects how the log behaves in service. Builders who match the sawing pattern to the intended use get more predictable results. The full breakdown of lumber dimensions, nominal versus actual sizes, shows how thoroughly the industry has standardized these relationships for boards.
Why the sawing pattern matters
A quartersawn piece moves less across its face than a flat-sawn piece, which is why quartersawn stock is preferred where stability matters. For a log wall, the pattern influences how the log checks and settles, so it belongs in the conversation with the supplier.
Questions to Ask Before You Pay for a Log Package
No single standard governs finished log diameters, so the supplier is the only source of truth. Before ordering, work through a short checklist and get the answers in writing.
The supplier checklist
- What is the exact finished diameter of each nominal size after milling and drying?
- Which species, initial moisture content, and sawing pattern does the quote assume?
- What tolerance does the mill hold on the finished diameter?
- How does the finished size affect wall height, corner joinery, and window and door clearances?
- Does the contract price by the nominal or the actual size?
The last question matters most. If the contract prices by the nominal foot but delivers actual dimensions, the delivered volume is smaller than the paid volume. The gap is not unique to logs: nominal plywood sizes and actual measurements diverge by fractions of an inch, so panel thickness must be verified before you design around it.
How the Finished Diameter Affects Height and Clearances
A half-inch difference on a single log becomes several inches across a twenty-course wall. Multiply the shrinkage by the number of courses, and the wall height, the rough openings for windows and doors, and the chase space for plumbing and wiring all shift. Designers who plan with the actual diameter from the start avoid the scramble at framing time.
Accounting for the difference in design
Ask the manufacturer what a 12-inch nominal log actually delivers in finished size and what impact the difference has on height and clearances in your design. Adjust floor-to-floor heights, header sizes, and insulation details before the package ships, not after the logs land on the pad.
The same discipline applies to every material that carries a nominal label. Verify plywood thickness explained through nominal and actual measurements before selecting panels, confirm lumber sizes before cutting, and treat every supplier nominal number as an invitation to ask what the actual number will be.