For uniform beads, a length calculation gives an estimate. A seller’s actual bead count is preferable because knots, packing gaps and strand ends use length too.
Estimated bead count = usable bead length in millimetres รท bead length along the hole in millimetres. Round down for the number that fits. For a purchase, divide the required bead count by the listed beads per strand and round up.
Worked example
A 400 mm usable strand of uniform 8 mm beads has room for approximately 50 beads. A project requiring 120 beads therefore needs at least three such strands. That is an arithmetic example, not a promise of the count in an unmeasured retail strand.
Measure the bead along the threading direction. Round, flat, irregular and faceted beads do not necessarily use their advertised widest diameter along the cord. Subtract clasp and spacer length when calculating a finished bracelet or necklace, and allow for rejects or replacements separately.
For exact colour matching, buy sufficient material from one lot. See bead diameter and hole-size checks before ordering.