Scale dimensions from boat plans for lofting or model building.
Common half-inch scale plan
Creating a model from a full-size boat
Converting metric plans
e.g., inches, mm
e.g., 24 for 1/2"=1'
e.g., 1 for full size
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Download NowLofting is the craft of turning a small lines drawing into full-size, fair curves on the loft-floor or directly on the boat, and the whole discipline rests on scaling every dimension from the plan's scale up to the size you need. A single mis-scaled offset throws a frame out of fairness and the error runs all the way through the hull.
The scale factor between drawing and reality is the ratio of the target scale to the plan scale, and multiplying any measured plan dimension by that factor gives the true size. Consistency of units and a careful factor are what keep a fair curve fair when it grows from a few inches on paper to feet on the floor.
Enter the plan scale, the target scale and a dimension measured on the drawing, and the calculator converts it to full size, so you can loft offsets, stations and lengths without mental arithmetic introducing a systematic error.
Scaling Factor = Target Scale ÷ Plan Scale; Target Dimension = Plan Dimension × Scaling Factor| Plan Scale | Meaning | Factor to Full Size |
|---|---|---|
| 1 in = 4 ft | Body plan / sections | 48× (in to in of feet) |
| 1 in = 8 ft | Profile / sheer plan | 96× (in to in of feet) |
| 1:10 | Metric general | 10× (same units) |
| 1:25 | Metric reduced | 25× (same units) |
Common lofting scales and their factors.
Formula verified against standard naval-architecture references · Updated October 2026
Find the scaling factor as the target scale divided by the plan scale, then multiply every measurement taken off the drawing by that factor. A plan at one inch to four feet means an inch on the paper is four feet in reality, so a 2.75 inch offset becomes eleven feet. The calculator does this conversion for any scale pair so you apply one consistent factor and avoid arithmetic slips.
Full size wherever you can, because that is what you build to and it exposes unfairness a reduced loft hides. When space or a first check demands, loft at a convenient reduced scale such as half or a metric fraction, verify fairness, then scale the same offsets up once. The key is consistency: choose a factor and apply it uniformly to every station.
Almost always a scaling or measurement inconsistency, where one or two offsets use a different factor or datum than the rest. Check that every point came from the same references and the same multiplier, and re-batten; fairness is judged by a spline through many points, so a single wrong-scaled value kinks the whole line and looks like a design fault when it is a lofting one.
It is the number you multiply a drawing measurement by to reach the real size, equal to the target scale divided by the plan scale. A 1:25 plan lofted at full size has a factor of 25; an inch-to-four-feet plan converted to inches has a factor of 48. Fix this one number first and every subsequent conversion is a simple, error-resistant multiplication.
Yes, but treat unit conversion as part of the scaling factor and apply it consistently. Compute the factor from the plan's metric scale to your target imperial dimension once, verify it against a known dimension, and use it for every offset, because mixing unit systems mid-loft is precisely how a whole hull is thrown out of proportion.
Enough that a batten faired through them cannot hide an error, typically several points along each curve at each station, with more near sharp changes like the turn of the bilge and fewer in gently run regions. The goal of lofting is to catch unfairness early, so density where curvature is high and a consistent datum matter more than raw count.
Long straight edges or battens for fairness, a well-struck datum centreline and waterline, a reliable scale or divider set, and something to compute the factor without mental arithmetic, which is exactly the role of this calculator. Precision comes from consistent references and checking the spline through many points, not from a single careful measurement.