Yield, recovery and conversion factors: what the number means

Why yield decides the economics

In most sawmills the logs are the largest single cost. A mill that turns a bigger share of each cubic metre of logs into saleable boards, or into more valuable boards, earns more from the same wood supply. That is why yield and recovery sit on every manager's dashboard, and also why the numbers are often compared unfairly.

Volume yield

Volume yield (volume recovery) is sawn volume divided by the log volume that produced it:

volume yield (%) = sawn volume (m3) / log volume (m3) x 100

Three choices change the answer without any change in mill performance:

• State of the lumber: rough green, kiln-dried rough and planed dry boards have different volumes, because drying shrinks the wood and planing removes material.
• How the log is measured: over or under bark, with or without allowances, by formula or by scanner. This is a convention, not a physical constant.
• What counts as a product: some studies count only the main product, others everything sawn and sold.

The FAO, ITTO and UNECE (2020) publication illustrates the first point with an illustrative example for construction softwood: rough green 1.57 m3 of roundwood per m3 of sawnwood (64 percent recovery), rough dry 1.63 (62 percent), and "sawnwood shipped" 2.08 (48 percent). The source calls this an example, not country data, and the 2.08 value was read from an extract with scrambled columns, so check it against the original before reuse.

The inverse: conversion factor

Statisticians use the reciprocal, the conversion factor: m3 of roundwood per m3 of sawnwood.

yield = 1 / conversion factor

The same publication reports 1.59 for coniferous sawnwood in Germany (Europe) and 2.04 for the United States (North America). The inverses are about 63 and 49 percent. These national factors depend on log measurement and lumber state, and the basis of each was not verified, so the gap between 63 and 49 percent is not a performance ranking of the two countries.

North American measures

North American mills, especially in the West, historically used board-foot log rules such as Scribner. This produced measures that are unfamiliar in Europe:

Keegan and colleagues (2010, western United States) argue that overrun is imprecise because log rules do not reflect real solid-wood volume, so LRF and CLR are more reliable. A CLR of 0.50 means 50 percent in green-lumber terms.

Value yield

Value yield asks how much money a cubic metre of logs produces:

value yield = (lumber revenue + by-product revenue) / log volume

It rewards sawing patterns that give wider, longer or higher-grade boards even when volume yield falls, and it rewards a sensible by-product outlet for chips, sawdust and bark. Volume and value yield can pull in opposite directions.

Worked example

A mill saws 100 m3 of logs and obtains 52 m3 of sawn boards.

• Volume yield: 52 / 100 = 52 percent.
• Conversion factor: 1 / 0.52 = about 1.92 m3 of logs per m3 of boards.
• If board revenue is 30,000 and by-product revenue 2,000 (currency units, invented for illustration), value yield is (30,000 + 2,000) / 100 = 320 per m3 of log.

Now a German-style conversion factor of 1.59 means 1 / 1.59 = 0.629, about 63 percent. A US factor of 2.04 means 1 / 2.04 = 0.490, about 49 percent.

Checklist: before comparing two yield figures

• Same unit (volume yield, conversion factor, LRF, CLR)?
• Same lumber state (green, dried, planed)?
• Same log measurement (over or under bark, method)?
• Same log size distribution and species?
• Volume or value, and at which prices?

Key points

• Volume yield = sawn volume / log volume; conversion factor is its inverse.
• The lumber state and the log measurement convention change the figure without any change in performance.
• Germany 1.59 (about 63 percent) and USA 2.04 (about 49 percent) are national factors with unverified bases; they are not a ranking.
• LRF and CLR are North American cubic-based measures; overrun is imprecise.
• Value yield can rise while volume yield stays flat or falls.