Just-in-Time Inventory Calculator

What JIT saves, and what it risks.

Compare just-in-time inventory against a traditional buffer, weighing carrying cost saved against stockout risk added.

Written and maintained by Mohit PatelLast checked August 4, 2026How we build these

Currency

Current average inventory

Inventory under JIT

Annual carrying cost

Extra stockouts expected per year

Cost per stockout

Net annual benefit of JIT

$950

the savings outweigh the risk

Carrying cost saved$13,750

Expected stockout cost− $12,800

Capital freed$55,000

Break-even stockouts per year4.3

The savings are steady and the failures are concentrated, make sure supply reliability actually supports the assumed stockout rate.

How the Just-in-Time Inventory Calculator works

Just-in-time minimises stock by taking delivery as close as possible to the point of need. It frees a great deal of cash and removes almost all margin for error: the savings are certain, the risk is occasional and large, and the comparison is worth doing explicitly.

Also known as: JIT inventory calculator · lean inventory calculator · minimum stock holding

The maths behind it

Just-in-time aims to hold only the stock needed for imminent demand. The calculation is what stock the system genuinely requires: demand during the replenishment interval plus a much smaller buffer, given very short and very reliable lead times.

JIT is not a formula so much as a set of preconditions. It works when lead times are short, supply is reliable, demand is stable, and the cost of a stoppage is manageable, and it fails badly when any of those does not hold.

Putting numbers to it

The example product at a three-week lead time needs 154 units at the reorder point. Move to a local supplier delivering in two days with high reliability, and lead time demand falls to 11 units with a buffer of perhaps 8, a reorder point of 19.

Average inventory falls from 174 units to perhaps 30, releasing $2,592 of capital and most of the storage space.

The cost is order frequency and unit price. Twice-weekly deliveries at $85 an order is $8,840 a year against $631, and a local supplier commonly charges 8% to 15% more per unit, $3,000 to $5,600 here. JIT wins the capital argument and loses the operating one on this product by a wide margin.

Where it is unreliable

JIT moves inventory rather than eliminating it. The stock still exists; it sits with the supplier, who prices it into the unit cost. The saving is real for the buyer and the total system holds much the same stock.

It also concentrates risk. A supply chain with no buffer transmits every disruption immediately, which is what made JIT operations the worst affected during the shipping disruptions of the early 2020s. Businesses that had buffers kept selling.

How to act on this

Apply it selectively rather than as a philosophy. High-value, predictable, locally-sourced items with short lead times are good candidates; anything imported, seasonal or erratic is not, and applying one policy to both is the usual failure.

Then price the disruption scenario explicitly. If a two-week supply interruption would cost more than a year of carrying the buffer, the buffer is cheap insurance and the JIT calculation has ignored the largest term.

What JIT actually requires to work

The Toyota system that JIT descends from rested on conditions most businesses do not have: suppliers located nearby, long-term relationships with shared planning, very high quality so that no buffer was needed for defects, and stable predictable demand.

Removing stock without those conditions does not produce JIT, it produces a fragile version of what was there before. The stock was absorbing variability, and taking it away without reducing the variability simply exposes it.

The useful part of the philosophy for a smaller business is the diagnostic rather than the target: every unit of buffer stock exists to absorb some specific uncertainty, and identifying which one: lead time, quality, demand, or record accuracy, points at what to fix. Fixing the underlying variability lets the stock come down safely, which is the opposite sequence from cutting stock and hoping.

Where to go next

The Just-in-Time Inventory question rarely arrives on its own. These are the ones that usually come with it:

Inventory Carrying Cost Calculator — The annual cost of holding stock, itemised.
Stockout Cost Calculator — What running out actually costs you.
Safety Stock Calculator — The buffer that absorbs demand and supply variability.
Etsy Fee Calculator — Every Etsy fee on one sale, itemised.

Not financial advice. Marketplace fees change, and they vary by country, plan and seller status. Every rate here is an editable default, not a quoted price, check the platform's current fee schedule before you price a product against it. This is not tax or business advice.

Frequently asked questions

What does JIT actually save?

Carrying cost on the inventory no longer held, typically 20-30% of that value annually, plus the capital freed for other uses. On £100,000 of stock removed, that is £20,000-30,000 a year plus whatever the cash earns elsewhere.

What does JIT risk?

Everything depends on supply reliability. One late shipment, one port delay, one supplier problem becomes an immediate stockout because there is no buffer. The savings are steady; the failures are concentrated and expensive.

Is JIT suitable for a small ecommerce business?

Rarely in full. It needs highly reliable short-lead-time suppliers, which small buyers seldom command. A partial approach, lean on predictable fast movers, buffered on volatile or long-lead items, captures much of the benefit with far less exposure.

How do I decide?

Compare annual carrying cost saved against expected annual stockout cost at the leaner level. If supply is reliable and stockout cost is modest, JIT wins. If either fails, the buffer is cheap insurance.