LDK stands for litres degrees kilograms. Sometimes you will see ltº/kg used. It is a measure of how much extract the brewer gets out of the malt. It is very easy to calculate: multiply the volume in litres by the last three digits of the specific gravity, and then divide by kilograms of malt used.
The US version of this measurement is points per pound per gallon. This measurement is seen a lot in American amateur brewing literature.
For example you have a total of 24 litres of wort at a gravity of 1050 brewed for 4.8 kgs of malt then:
LDK = (24 * 50)/4.8
250 is a good LDK figure for an amateur setup.
Extract efficiency given as a percentage figure is simply the comparison of your LDK figure against that of a standardised laboratory mash. For pale malts – pilsner, lager, ale etc, the laboratory LDK typically ranges 305 to 315.
Taking 310 as an average laboratory LDK then 250 is about 81% efficiency (250/310 * 100/1).
Some malt analyses will give the LDK figure – it is the figure for Hot Water Extract. Other analyses show extract as a percentage. There are often two figures, the one we need is dry basis fine grind (DBFG). Multiply by 386 to get the LDK figure. Here the “dry basis” means that the malt has all its moisture removed before it is weighed and mashed. Typical moisture content is around 4%.
LDK will vary with equipment, procedures, and grain bill. Crystal and roasted malts will have lower LDKs than base malts.
As long as your LDK is over 200 it doesn’t really matter what it is. High efficiency does not mean high quality. Trying to take too much from the malt risks leaving the beer tasting harsh and astringent.
Instead, you should aim for a consistent LDK figure brew to brew. If you are relatively new to grain brewing then measuring LDK will help you develop more consistent brewing procedures. With consistent procedures you can work more effectively to improving your brewing and beer.
If you have a consistent LDK figure you can plan your brews more easily, using it to work out how much malt you will need for the desired volume and gravity. It can also help you assess recipes, and modify them if necessary.
There are several points in the brew where you can measure volume and gravity. The first is once the full volume of wort has been collected in the kettle and is coming up to the boil. The second is at the end of the boil and the wort is still. The best way to measure volumes in the kettle is to use a one metre stainless ruler as a dipstick. When measuring the volume of near boiling wort multiply by 95% to compensate for the expansion caused by the heat. Gravity samples must be cooled to 20 degrees.
If you measure the volume in the fermenter you will also need to measure the amount of kettle wastage, and losses to transfers and heat exchanger (if applicable). These should be reasonably consistent brew to brew, perhaps varying somewhat with the amount of hops used. Usually this varies around 1 to 2 litres depending on equipment and procedures.
When assessing a recipe, for the usual 20 to 22 litres fermenter volume assume system waste of 1.5 litres. Let’s say the recipe calls for 4.4 kg of malt to give 22 litres at a gravity of 1048.
(22 + 1.5) * 48/4.4 = 256
An LDK of 256 indicates very good mash efficiency – 82.5%. If your LDK is averaging around 230 then you would need to increase the amount of malt by about 10 to 12% to get 22 litres of 1048 wort in the fermenter.
Working out the malt required using your LDK of 230:
(22 + 1.5) *48/230 = 4.9 kg
LDK is a useful measurement which will help you to brew more consistently, and more accurately.