The number of squares in a blanket comes from two decisions: the size of a finished square and the size of the finished blanket. Joins and borders can add enough width to matter, so include them before making a large stack of motifs.
Measure a finished square
Use a sample that has received the same care and finishing treatment as the planned blanket. Measure both directions. If the motif is not approximately square, a square-grid calculator will be a poor model; work out rows and columns using the actual width and height instead.
Check several samples if they are available. A 0.5 cm difference may look small in one motif, but it adds 5 cm across a run of ten. Consistency matters more than forcing every motif to match an attractive round number.
Count spaces between squares
Six 15 cm squares placed directly beside one another make 90 cm. There are five joins between them. If each join adds 0.5 cm, the assembled width is 92.5 cm. A 3 cm border on each side brings the total to 98.5 cm.
Join width means additional width, not the physical thickness of the yarn. Measure two joined squares and compare their total width with the sum of the individual widths. If the joining technique adds effectively no space, use zero. If it pulls the fabric in, do not enter an invented positive gap; use the measured assembled dimensions to refine your plan.
A 100 × 140 cm target
With 15 cm squares, 0.5 cm joins and a 3 cm border, the blanket planner suggests 6 columns by 9 rows. That is 54 motifs and an estimated finished size of 98.5 × 145 cm. The difference from the target is the cost of choosing whole motifs.
Eight rows would give 129.5 cm in length. Neither option is “wrong”; they answer different preferences. Choose whether you prefer a longer blanket, a shorter one, a different border or a different square size. A target dimension is not a guarantee that an exact arrangement exists.
When maximum size matters
Our planner chooses the nearest whole-square layout, which can exceed the target. If the finished item must fit within a specific space, compare the estimated output against that maximum. Test a smaller target until the chosen arrangement fits, or calculate the maximum columns by rounding down instead of to the nearest whole number.
Do not solve a dimensional mismatch by stretching your work beyond its natural shape. It may relax differently after use or washing, and the numerical plan will no longer describe the fabric accurately.
Plan a manageable batch
Write a small inventory before starting: six columns, nine rows, 54 squares, chosen colours and joining method. Use a labelled row-by-row layout so you can check completed pieces against the plan. A three-colour diagonal repeat is convenient for a study, but you can arrange the actual motifs differently on a flat surface before joining.
The preview counts whole-square colours. If each motif uses three colours in different rounds, keep a separate round-level key. Save or print the layout and make a note of your real yarn shade names, because screen colours cannot identify a dye lot.