“How many balls for a blanket?” has no single answer. The dimensions, stitch, yarn and maker all affect the result. A measured sample lets you replace a generic shopping guess with an estimate you can check and revise.
Start with a matched area and weight
Make a rectangular sample in the same fabric as the main blanket. Measure its complete dry width and length and weigh that same piece, without long loose tails. Record the yarn label’s ball weight and length. If the scale is not sensitive enough for a light sample, make a larger piece so a small weighing error accounts for a smaller share of the total.
Suppose your complete 10 × 10 cm sample weighs 6 g. Its area is 100 cm², and it uses 0.06 g per square centimetre. A 100 × 140 cm blanket has an area of 14,000 cm². At the same density, the main fabric estimate is 840 g.
Make the allowance explicit
Now separate the arithmetic from your planning choices. If you choose a 15% allowance, multiply 840 by 1.15 to get 966 g. That percentage is an example, not a universal crochet rule. A simple rectangle, a blanket with frequent colour changes and a heavily bordered motif blanket have different sources of extra consumption.
The yarn calculator makes the allowance editable. Record why you chose it: spare yarn for repairs, tails, uncertain measurements or a small margin for tension changes. Estimate a substantial border or a different joining fabric separately rather than expecting an arbitrary percentage to cover it.
Convert the estimate to whole balls
If the yarn comes in 100 g balls, 966 ÷ 100 is 9.66, which rounds up to 10. If each ball contains 200 m, those ten balls contain 2,000 m. The estimated consumption is 1,932 m; the difference is the remainder created by rounding to whole balls.
For another yarn, never reuse the metres-per-gram ratio automatically. A ball weighing the same amount can have a different length. Use the new yarn’s label and a new sample. See the substitution guide for a separate length-based comparison when following a published pattern.
Understand the uncertainty
Imagine that the sample actually weighs 5 g rather than 6 g. The base estimate drops from 840 g to 700 g. This is why a precise-looking calculator answer should not be mistaken for a promise. Both measurements and the assumption of consistent fabric matter more than the number of decimal places.
You can make uncertainty visible by calculating a low and high case using the range you measured in several comparable samples. If those cases differ by multiple balls, improve the sample before committing to the larger project. Do not average together samples made from different stitches or yarns.
Plan colours separately
For identical single-colour motifs, weigh one representative finished square of each construction and multiply by the number needed. Account for joining yarn separately. For multi-round colourwork, a square count alone does not reveal how many metres each colour needs: an outer round can use more than an inner one.
Keep the labels and your calculation with the project. Once a measurable section is complete, compare actual use with the original estimate. That early check is more useful than discovering a mismatch at the final border.