Kanban sizing and cards

How the kanban cards are sized

A kanban loop holds a fixed number of containers, each with its card. When a container is used, its card goes back and authorises making or fetching one more. The number of cards caps the stock of the part and, through Little’s law, its lead time. The formula below is the usual one, not a standard; it sizes a loop for an average demand.

Formulas

• Cards N = ⌈D · L · (1 + α) / C⌉, at least 1: D the demand per day, L the replenishment lead time in days, α the safety factor and C the quantity per container. The exact quotient is shown next to the rounded number.
• Lead time in hours is turned into days with the working hours per day: L = hours / working hours per day.
• Pieces in the loop (all containers full) = N · C; days of demand = N · C / D. The chart splits them into the lead time, the safety on top and what rounding up to whole containers adds.

Worked example (the tool’s example)

1. Bracket BR-1040: 480 pieces a day, 1.5 days to make and bring a container of 60, safety 10 %: 480 × 1.5 × 1.1 / 60 = 13.2, so 14 cards; the loop holds 840 pieces, 1.75 days.
2. Screw M6 × 16: 3200 a day, 2 days, boxes of 1000, safety 20 %: 3200 × 2 × 1.2 / 1000 = 7.68 → 8 cards.
3. Label roll LB-7: 240 a day, 3 days, rolls of 500: 1.58 → 2 cards. All five parts need 44 cards.

Common mistakes

• Taking only the production or transport time as the lead time: waiting for collection, batching of cards and queues are part of the loop.
• Sizing with the peak demand all year: size for the average and recalculate when the demand changes.
• Rounding down to save stock: the loop then runs dry every time the lead time is reached.
• Leaving the number of cards unchanged for years: re-size it at every change of demand, lead time or container.

Input and files

• Numbers may use a decimal comma or point. The optional columns (Description, Safety (%), Type, From, To) are found by their header names; a part without a safety factor takes the field’s default.
• Print cards: one card per container, eight to an A4 sheet with cutting lines, with a QR Code or Data Matrix made by FactoKit’s own encoders (check a printed card with your scanner). The blank card (fillable PDF, Word, Excel) is for loops sized elsewhere.
• Excel export: one row per part with the cards, pieces and days as formulas over the inputs.
• Files: CSV, TSV and TXT are read as if pasted (UTF-8, UTF-16, otherwise Windows-1257). XLSX: choose the sheet and the columns or a cell range; formula cells use the value Excel last saved, dates are kept as text. Files up to 20 MB are read in this browser only and never uploaded.
• XLSX files are read with SheetJS Community Edition 0.20.3, Apache License 2.0 (license text at the end of this file).

Questions

How many kanban cards do I need?

The usual formula is N = demand per day × replenishment lead time in days × (1 + safety factor) / quantity per container, rounded up. The lead time is the whole loop: from the moment a card is released until its full container is back at the point of use, waiting and transport included.

What safety factor should I use?

It covers the variation of demand and lead time that the average does not. 10 to 20 % is often used to start; lower it as the loop proves stable. A large safety factor hides problems such as unreliable supply instead of fixing them.

What is the difference between a production and a withdrawal kanban?

A production kanban authorises the supplying process to make one container; a withdrawal (move) kanban authorises moving one container from a store to the point of use. Both are sized with the same formula, with the lead time of making or of fetching.

Why should a loop have at least two cards?

With one card the only container must be used up before it is sent for refilling, so the consumer waits for the whole lead time. With two or more, one container is in use while the others are being refilled.