A kanban card is a reorder signal you can hold. In a two-bin system, each material lives in two bins of the same size, each with a card. You use from one; when it's empty, its card goes to whoever orders, and the full bin carries you until the delivery arrives. Nobody counts anything to know when to reorder: an empty bin is the count. The work is in sizing the bins, so the second one lasts as long as the supplier takes, plus a few days for safety. Below is how to size a bin, a filled-in example for a candle maker's six materials, how the cards move, when it stops working, and a free template that sizes the bins and prints the cards.
The short version
- Two bins of each material, each with a card.
- Use from one; when it's empty, send its card to be reordered.
- A bin holds enough for the supplier's lead time plus safety days.
- Round up to whole cases, boxes or rolls, so a bin is a real container.
- The reorder quantity is one bin.
- Check the sizes when use changes, before the season does it for you.
- TaroStack does the same sum from how fast stock moves, and puts it on a reorder list in days of cover. More below.
How to size a bin
A bin has to last from the moment the other one empties until the delivery is on the shelf:
Bin quantity = used a day × (lead time in days + safety days), rounded up to whole containers.
The lead time is how long the supplier takes, door to door, not what the website says; how to measure it is in lead time calculation formula. The safety days cover a slow delivery or a busy week; how many you need is the same question as in safety stock formula.
A filled-in example: a candle maker's shelf
Lehua Glow, a made-up candle maker on Oʻahu I use in these examples, pours about 40 nine-ounce candles a day and runs its six main materials on two bins:
| Item | Used a day | Lead time | Safety | Container | Containers a bin | Bin quantity | A bin lasts |
|---|---|---|---|---|---|---|---|
| Soy wax flakes | 19 lb | 7 days | 3 days | case of 50 lb | 4 | 200 lb | 10.5 days |
| Wick, pre-tabbed, for 9 oz jar | 40 | 10 days | 5 days | box of 100 | 6 | 600 | 15 days |
| Amber jar, 9 oz | 40 | 14 days | 7 days | case of 12 | 70 | 840 | 21 days |
| Lid for the 9 oz jar | 40 | 14 days | 7 days | box of 100 | 9 | 900 | 22.5 days |
| Fragrance oil, plumeria | 6 oz | 7 days | 4 days | bottle of 16 oz | 5 | 80 oz | 13.3 days |
| Label, 9 oz jar | 40 | 10 days | 5 days | roll of 500 | 2 | 1,000 | 25 days |
The wax is worked out as 19 pounds a day × (7 + 3) days = 190 pounds, rounded up to four 50-pound cases, 200 pounds a bin. The jars come from farther away, so they need the biggest bins: 21 days of jars, 70 cases, which is why the jar "bin" is a section of rack, not a tub. A bin doesn't have to be a box; it's a set amount in a marked place.
How the cards move
- Use from bin A. Bin B stays full and closed.
- When bin A is empty, its card goes on the order board.
- Whoever orders places the order for one bin's worth, and writes the date on the card.
- Use from bin B while the order is on its way.
- When the delivery arrives, it fills bin A, and the card goes back with it.
If bin B runs out before the delivery, the bin was too small, or the supplier was late: either the use went up, the lead time is longer than you thought, or the safety days were too few. Change the size on the sheet and print a new card.
Kanban, a reorder point, and a par level
A two-bin kanban is a reorder point you can see: the reorder point is one bin, and the empty bin says you've reached it. A par level is different, a level you top up to on a schedule. When each fits, with examples, is in par level vs reorder point, and the same arithmetic written as a formula is in reorder point example.
Get the template
Download the kanban card template. Two sheets: an item list where you type how much you use a day, the lead time, safety days and how much a container holds, and the containers a bin, bin quantity and days a bin lasts fill in; and a sheet of printable cards, Bin A and Bin B for each of the first six items, filled in from the list. It's an ordinary Excel file with no macros, and it works in Google Sheets, LibreOffice and Numbers.
Where this stops working
Two bins work for a few dozen materials used at a steady rate. They stop working when use swings with the season and nobody resizes the bins, when a card gets lost and the reorder never happens, when two people both think the other ordered, or when a supplier's minimum is bigger than a bin. A card says when to order; it doesn't say what's already on order, or that the jars are late.
How TaroStack does it
TaroStack does the same sum from how fast stock moves.
A reorder list in days of cover. Every material shows how many days of use are left on the shelf, from how fast it has been used, and the reorder list puts the ones running short at the top, so a busy December moves the jars up the list without anyone resizing a bin.
Reorder points from real use. TaroStack suggests a reorder point for each material from how stock actually moves, and each supplier carries its lead time and minimum, so the order is at least what the supplier will take.
Late is visible. Purchase orders show what's already on order, and lead times are checked against how long deliveries really take, so the jars that always arrive four days late stop surprising you. Counting with a phone or a scanner keeps the numbers honest between deliveries.
| The job | Two bins and cards | In TaroStack |
|---|---|---|
| When to reorder | The bin is empty | Reorder list in days of cover |
| How much to keep | Sized on a sheet | Suggested from how stock moves |
| What's already on order | The card on the board | Open purchase orders |
| A supplier that's always late | Noticed eventually | Lead times against real deliveries |
Reorder lists, suppliers and purchase orders are on every plan, from $49 a month. The first 30 days are free, with no credit card. If you'd like a hand setting up, ask, and we'll do it with you.