Ink usage calculation: how complicated is it

How does a printer calculate the ink usage of a specific job? It’s an important question, if only because it should be known to stick a price on a job. It used to be done by applying the SPANKS formula. Today, it isn’t that straightforward anymore.

I decided to ask the question in three professional printing groups on LinkedIn for a change, and not my usual contacts at the companies that manufacture the printing devices (Heidelberg, Man Roland, VUTEk, and Agfa. I was surprised to find that only two people seemed to know what I wanted. Carl Smith, business development manager with British company FFEI, and Michael Jahn, Implementation and Support Specialist at US-American Datatech Smartsoft.

Both knew what I meant with “SPANKS”, and both offered a good deal of insight with how one calculates ink usage. It turns out the answer is not as simple as what the question suggests.

Mr. Jahn answered that SPANKS is a formula used to prepare production cost estimates for “Estimating Ink Quantities” — an acromym of which the letters stand for what is taken into account when calculating the amount of ink used in a particular job:

• Stock (type of paper/substrate)
• Process (type of printing process)
• Area (printed Area of each sheet in square meters)
• Number (total Number of sides to be printed on)
• Kind (is this printed piece light type, bull bleed color process, etc..)
• Specific gravity (density of the ink(s) used)

He explains numbers are then assigned to each of the SPANKS factors. “It is (obviously) not an exact science, sort of a ‘rough estimate’ sort of tool, with a huge range of error between different people,” mr. Jahn commented. The SPANKS formula can be downloaded from PrintPlanet — when you have a login ID and password for that site.

Some ink manufacturers offer a slightly more accurate version because they understand the behaviour of the ink on some select media isn’t always identical, said mr. Smith. “Basically for any given job you need to know the area coverage of each separation. Then you need to understand the amount of ink which will be used for 100% coverage. Then its case of number of sides, volume etc.,” he said.

Mr. Smith withheld five variables:

1. The print process, media, ink and press setup all matter and affect the result.
2. If colour management is used the area coverage will change due to GCR. So you must calculate the ink usage with data just before it is imaged. On some digital devices this is hard because the device may use internal colour management to support additional channels like cyan light, orange, purple etc. This is why many commercial solutions that calculate from PDF can be completely wrong.
3. In the case of inkjet technology it is much more complex as touched on in this EFI article. Many inkjet heads such as Xaar 1001 have several gray levels which can be tuned to deliver a controlled droplet of 4-40 picolitres. Therefore to calculate the ink usage you must calculate each of the 8 gray levels independently using the screened 8bit files. If you have a head array — e.g more than one head used in unison for greater speed or resolution — then it becomes more complex again.
4. Some inks cost more than others such as white and this needs to be factored in on final cost.
5. If your printing variable data then you need to average.

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