Kanban & Reorder Calculator

What this template helps you do

Use straightforward formulas and worked examples to size Kanban card counts, calculate reorder points (ROP), and estimate safety stock based on demand and lead-time variability. Includes quick checks, common mistakes to avoid, and practical layout suggestions for a basic pull board or two-bin system.

When to use Kanban vs. reorder point

Kanban (card-driven pull) works well when you can standardize container sizes, reliably replenish in predictable batches, and want a visual signal. Reorder point logic (ROP) is useful when exact container sizing is impractical or when you prefer a calculated trigger to create a purchase or production order. This template gives the math and guidance for both approaches.

Key variables and notation

• D = average demand per unit time (e.g., units per day or week)
• σd = standard deviation of demand per unit time
• L = average lead time (same time units as demand, e.g., weeks)
• σL = standard deviation of lead time (if significant)
• C = container or replenishment lot size (units per Kanban container)
• z = safety factor for desired service level (z ≈ 1.28 for 90%, 1.65 for 95%, 2.33 for 99%)

Basic Kanban card count (no safety stock)

When demand and lead time are stable:

Number of Kanban cards = (D × L) / C

Round up to the next whole card. If partial cards remain, they represent an extra container you should include.

Safety stock for demand variability (approximate)

To protect against variability in demand (and optionally lead time), estimate the standard deviation of demand during lead time and multiply by the z-factor:

σ(D during L) ≈ sqrt(L × σd² + D² × σL²) (if both demand and lead time vary and are independent)

Safety stock = z × σ(D during L)

When lead time variability is negligible, simplify to:

σ(D during L) ≈ sqrt(L) × σd

Kanban cards including safety stock

Convert safety stock into equivalent cards and add to base card count:

Cards for safety stock = ceil( safety stock / C )

Total Kanban cards = ceil((D × L) / C) + Cards for safety stock

Reorder point (ROP)

When you prefer a numeric trigger rather than cards:

ROP = D × L + Safety stock

Place reorder when on-hand inventory drops to or below the ROP.

Worked example — simple Kanban (step by step)

Assume:

• D = 100 units per week
• L = 2 weeks
• C = 25 units per container
• σd = 20 units/week
• Assume σL is negligible
• Target service level ≈ 95% (z = 1.65)

Base cards = (100 × 2) / 25 = 200 / 25 = 8 cards

σ(D during L) ≈ sqrt(L) × σd = sqrt(2) × 20 ≈ 28.3 units

Safety stock = 1.65 × 28.3 ≈ 46.7 units → roughly 47 units

Cards for safety stock = ceil(47 / 25) = 2 cards

Total Kanban cards = 8 + 2 = 10 cards

If using ROP: ROP = D×L + safety stock = 200 + 47 = 247 units

Multi-SKU and mixed-container considerations

• Size containers consistently when possible. Mixed-size containers complicate visual Kanban counts and increase administrative work.
• When many SKUs share a storage location, track cards or electronic signals per SKU to avoid cross-use of containers.
• For slow-moving items, consider minimum-card rules (e.g., at least 1 card) and periodic review rather than frequent card counts.
• Group SKUs with similar demand patterns for simpler review and replenishment batching.

Quick checks to avoid overstock or stockouts

• Check container size C: if C is much larger than average usage during L, Kanban rounds up and you’ll see high on-hand inventory.
• Review σd and σL every 3–6 months — increases in variability should increase safety stock/cards.
• If frequent expediting still occurs, measure actual lead-time distribution and use it in the σ(D during L) calculation rather than assuming constant L.
• Use a min/max review for very low-volume SKUs rather than complex card rules.

Pull-board / two-bin layout suggestions

Keep the visual system simple so anyone can act quickly:

• Columns or areas: Available / In Use / Empty (Signal) / In Replenishment / Incoming.
• A Kanban card in the Empty/Signal column should clearly show who to notify, what to order, and the container size.
• Use color-coding for priority SKUs or for items with long lead times.
• Place frequently used SKUs at eye level and keep the signal area highly visible.

Common mistakes and how to avoid them

• Using extremely large containers (C) to reduce card counts — this increases total inventory and hides problems.
• Ignoring lead-time variability — treat supplier reliability as a key input and include σL when it matters.
• Never updating parameters — demand seasonality, product changes, and supplier performance change over time.
• Over-complicating the system with too many exceptions — start simple, then add targeted adjustments where needed.

Practical next steps & checklist

1. Collect average demand (D) and standard deviation (σd) for the chosen time unit.
2. Confirm average lead time (L) and measure lead-time variability (σL) if possible.
3. Choose an appropriate container size (C) and a target service level (select z).
4. Compute base cards, safety stock, and total cards using the formulas above.
5. Implement the Kanban signal and monitor fill rates, stockouts, and expediting events for 4–8 weeks.
6. Adjust C, z, or card counts based on real-world performance and supplier conversations.

When to consider capability enhancements (tooling)

This template gives the formulas and guidance, but it is often more useful when paired with a lightweight interactive calculator or a saved worksheet so teams can input local D, σd, L, σL, C and see card counts and ROP instantly. Integrations that pull demand history and lead-time logs automatically reduce errors and keep calculations current.

Suggested monitoring KPIs

• On-time supplier delivery percentage
• Stockout occurrences per SKU per quarter
• Average inventory days or units by SKU
• Kanban signal-to-fulfillment lead time

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