How to use the kanban card quantity calculator
- Enter the daily demand of the process that uses the item and the replenishment lead time in days.
- Choose whether your safety allowance is a percent of lead time demand or a number of units, and enter it.
- Enter how many units fit in one container.
- Read the number of cards, the units in the loop and the safety stock they actually give. The table shows how the count moves if demand changes.
Formula
lead time demand = daily demand x lead time
cards = ceiling of (lead time demand + safety stock) / container size
safety stock = lead time demand x safety percent, or the units you enter
units in the loop = cards x container size
Cards are whole numbers, so the loop almost always holds slightly more than the demand plus safety stock you asked for. The tool shows the safety stock the cards really give.
A daily demand of 100
Demand is 100 units a day and replenishment takes 2 days, so 200 units are used while a refill is on its way. A 10% safety allowance adds 20 units, and with 20 units to a container the loop needs (200 + 20) / 20 = 11 cards, holding 220 units.
If demand rises 10% to 110 a day, the loop needs 12.1 containers, which rounds up to 13 cards. At 120 a day it needs 14. Switching to a safety stock of 30 units instead of 10% gives (200 + 30) / 20 = 11.5, so 12 cards.
How a kanban loop works
Each container carries a card. When a container is emptied, its card goes back to the supplier or the previous process, and that signals a refill. The number of cards caps the stock in the loop, so adding a card adds one container of stock. A bigger container or a shorter lead time lowers the number of cards you need.
For items you order from outside, compare with the EOQ Calculator and the Economic Production Quantity Calculator. The Little's Law Calculator links stock in the loop to flow rate and time.
Choosing the safety allowance
A fixed percent is easy to explain, but it protects less when demand varies a lot. If demand swings, size the buffer from the history of demand rather than a flat percent, and revisit it often. Cards can be added or removed as conditions change.
Assumptions and limits
- Demand is steady, and lead time covers the time from the card being released to a full container arriving.
- All containers hold the same number of units.
- The sensitivity table changes only the demand and keeps every other input.
- Changeover time, several items sharing a line and uneven orders are not modeled.
- The count is a starting point to be adjusted by watching the loop run.
Frequently asked questions
Why does the card count round up?
A container is the smallest unit you can add. Rounding down would leave the loop short of the demand during the lead time.
What is the effective safety factor?
It is the safety stock the whole cards give, as a share of lead time demand. It is at least what you asked for because the count rounds up.
Can I use hours instead of days?
Yes, if you use hours for both the demand per hour and the lead time. Keep the units consistent.
How often should the number of cards change?
Review it when demand, lead time or container size change, and after a stockout or after the loop sits full for long.