Two boxes of Duck shrink film sit side by side on Amazon.nl. The small one, 62 by 210 inches at EUR 39.05, claims 15 windows; the large one, twice the film at EUR 86,87, claims 10. The box count is not something you can compare with.

What you need to know

Duck is an American weatherproofing brand and Dutch rents are up 4,3% year on year, and in most rentals the windows are not yours to replace. Shrink film is the layer a tenant can actually add: it goes on the inside of the frame and traps a still layer of air behind it. The glass still feels cold in January, but the draught you feel while sitting beside the window stops. On Amazon.nl the two boxes are EUR 39,05 and EUR 86,87 today; the small one is sold and shipped by Amazon, with free delivery shown for Sunday 13 September.

Put the "covers N windows" line aside and work in area instead, because both boxes print their dimensions. Sixty-two inches by 210 is about 8,4 square metres, and 39,05 divided by 8,4 is EUR 4,65 per square metre. Sixty-two by 420 is about 16,8 square metres, and 86,87 divided by 16,8 is EUR 5,17. Same brand, same film, and the big box costs roughly a tenth more per square metre.

Now swap in your own numbers. Take a tape measure to one window, multiply its width by its height for that window's square metres, add up the ones you mean to cover, and divide by 8,4 to get the number of boxes. Leave slack: the film has to overhang the frame by a few centimetres on every side to grip, and what you trim off means the usable area is a little under 8,4. One more thing worth knowing before you commit: this is heat-shrink film, tightened with a hairdryer, and the frame has to be clean and dry when you apply it.Source 1

The small box, 62 by 210 inches, is about 8,4 square metres at EUR 39,05, sold and shipped by Amazon.
The large box, 62 by 420 inches, is about 16,8 square metres at EUR 86,87, shipped by a third-party seller.
The two window counts contradict each other: the box with twice the film claims to cover fewer windows.
The kit contains two rolls of clear shrink film plus the matching window sealant tape.
Fitting relies on hairdryer heat to shrink the film; the frame must be cleaned and dried first.
Platform rating 4.5 from 5.647 reviews, with both sizes sharing one review pool.

Hokimi tip: measure your coldest window and buy only enough to cover that one for a winter. Film changes how you use the window, because once it is on you cannot open that window to air the room, and that catches people out more often than the money does.

The link to this insulation kit is an affiliate link. The price is identical to going direct, and Hokimi may receive a small commission.