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Step 1: How many pieces fit on a sheet?
Add the bleed to every edge of the piece, take the press's gripper margin off the sheet, then try both orientations. Example: a 70 × 40 mm label with 3 mm bleed takes up 76 × 46 mm. On a 660 × 960 mm sheet with a 10 mm gripper, the usable area is 650 × 960 mm.
One way round: 960 ÷ 76 = 12 rows, 650 ÷ 46 = 14 rows, 168 labels. The other way: 960 ÷ 46 = 20, 650 ÷ 76 = 8, 160 labels. Take the better one: 168 labels per sheet.
Step 2: How many sheets? Don't forget waste
For 10,000 labels, 10,000 ÷ 168 = 59.5, so 60 sheets. But give the press 60 sheets and you won't get 10,000 labels. Waste comes from three places:
- Make-ready waste: sheets printed until colour and register are right. It doesn't depend on the job size; it is a fixed number for the press and colour count, say 50 sheets.
- Running waste: sheets spoiled during the run, as a percentage of the run, say 3%.
- Finishing waste: lamination, cutting, carton gluing and so on each add their own.
Step 3: How many kilos, and what does it cost?
In the example: 60 + 50 make-ready + 2 running waste = 112 sheets. A sheet weighs width × height × grammage: 0.66 m × 0.96 m × 90 g/m² = about 57 grams. 112 sheets × 57 g = about 6.4 kg.
At a paper price per kilo, multiply: that is the paper cost. Notice that on a small job the make-ready is almost half of the sheets. On a short label run it is the make-ready, not the quantity, that drives the paper cost.
Cartons: large flat size, heavy board
The calculation is the same for a carton, but the scale changes. A carton with a 300 × 420 mm flat size (the outer edge of the dieline) goes 2 × 2 on a 700 × 1000 mm sheet, gripper included: 4 cartons per sheet.
5,000 cartons need 1,250 sheets; with 100 sheets of make-ready and 5% running and die-cutting waste, about 1,413. On 350 gsm board one sheet is 0.7 m² × 350 g = 245 grams, about 346 kg in all. Board gets heavy fast, so even a small difference in sheet choice makes a big difference in cost.
The sheet that yields the most isn't the cheapest
When choosing a sheet, don't ask “how many fit”, ask “how much paper does each piece use”. The same carton also goes 2 × 2 on a 640 × 900 mm sheet: 4 cartons again.
On 700 × 1000 each carton uses 0.7 m² ÷ 4 = 0.175 m² of board; on 640 × 900, 0.576 m² ÷ 4 = 0.144 m². Same count, but on the smaller sheet every carton uses about 18% less board. Over 5,000 cartons, with the same waste, that is about 285 kg instead of 346 kg.
Cartons have one more condition: the grain direction has to suit the creases. A sheet with the wrong grain spoils the carton however cheap it is, so compare only sheets with the right grain.
At month end: how much paper went out?
The monthly total matters as much as each job's paper: what is left in stock, how much of which paper to order. Recording sheets and kilos for every job and adding them up at month end takes the guesswork out of paper orders.
One thing to watch: when the paper price changes, past jobs' costs should not. Keep records at the price on the day the job was done.