Inventory planning basics for ecommerce sellers

Reorder point: a timing trigger

A reorder point is the inventory threshold used to trigger a replenishment review. In this toolkit, it is demand during supplier lead time plus safety stock.

Average daily sales × supplier lead time + safety stock

Choose a demand rate that matches the decision: for example, shipped units, fulfilled units, or another internally consistent measure. Do not combine a short promotion period with a normal lead-time assumption without reviewing the result.

A reorder point answers when to review or place an order. It does not by itself decide the purchase quantity.

Small example: 12 units per day × a 20-day lead time + 60 units of safety stock = a 300-unit reorder point.

Common mistakes: using quoted transit time instead of full sellable lead time; mixing sales windows between periods; treating the reorder point as the amount to order rather than a trigger.

Safety stock vs. reorder point: buffer vs. trigger

Safety stock and reorder point answer different questions. Safety stock is how much extra you hold above expected demand; the reorder point is when to act — the stock level that starts a replenishment review. They connect directly:

Reorder point = average daily sales × lead time + safety stock

Small example: demand during a 20-day lead time is 240 units and your buffer is 60 units, so the reorder point is 300 units. When on-hand stock reaches 300, you review and order; the 60-unit buffer inside that threshold covers surprises.

Common mistakes: ordering the buffer quantity when the trigger is reached instead of recalculating a full order; setting a reorder point with no buffer at all; counting the buffer twice by adding it again inside the order quantity.

Both figures come from user-provided averages and maxima. They are planning estimates, not service-level guarantees.

Size the buffer, then set the trigger.

Safety stock: a buffer, not a guarantee

Safety stock is inventory held above expected demand to absorb variation in sales or lead time. The Safety Stock Calculator on this site uses a conservative maximum-minus-average planning method based on the inputs you provide.

That simplified method is not a service-level calculation and does not represent every safety-stock approach. Use comparable historical periods, and review whether exceptional promotions, stockouts, supplier disruptions, or new-product launches make the maximum and average values misleading.

No single safety-stock formula fits every business, SKU, or data-quality situation.

Lead time and demand rate: define the boundaries

For replenishment planning, lead time should cover the period from the point you make the purchase decision until stock is sellable for the channel you are planning. That may include supplier processing, transit, customs, receiving, quality checks, putaway, or marketplace availability.

Demand rate should use the same unit and stock pool as the inventory figure. If inventory serves several channels, the demand input should reflect the relevant combined demand.

Quoted transit time alone may not match the operational lead time that matters to a stockout decision.

Confirmed inbound inventory: timing matters

Inventory position can be useful for planning, but confirmed inbound inventory is not the same as sellable on-hand stock. It should only influence a future projection when the arrival date is confirmed enough for your policy and the receipt is expected to become sellable in time.

Track whether the inbound is produced, shipped, received, held for inspection, or delayed in the warehouse. A partial receipt or a delayed receipt can change the planning result.

Do not assume all open purchase orders prevent a stockout.

MOQ and case packs: orderable quantity constraints

Supplier minimum order quantities, case packs, and order multiples can make a calculated net need different from the quantity you can actually order. The Purchase Order Quantity Calculator shows its raw need before applying those constraints; the Reorder Planner includes the steps in its calculation snapshot and emphasizes the final recommended order.

Review whether a larger compliant order fits cash availability, storage space, shelf life, product obsolescence risk, and supplier capacity.

A mathematically valid order quantity is not automatically an approved purchase decision.

Small example: raw need 90 units, MOQ 100, case pack 24: the MOQ lifts the order to 100, then whole cases round it to 120 (5 cases of 24).

Common mistakes: treating MOQ as a target to buy up to; forgetting that rounding up increases cash and storage needs; ignoring order multiples larger than the case pack.

How to calculate days of inventory

Days of inventory (days of cover) converts your on-hand stock into time: how many selling days it would last at the current demand rate.

Days of inventory = on-hand units ÷ average daily sales

Small example: 180 sellable units ÷ 9 units per day = 20 days of cover. The figure moves every day as stock and demand change.

Common mistakes: counting reserved, damaged, or unsellable stock; pairing a holiday-season demand figure with off-season stock, or the reverse; assuming inbound stock arrives in time without confirming.

The result uses the on-hand count and demand rate you provide; actual sales vary day to day.

Turn days of cover into a stockout date, or see it in the combined planner.

Stockout date: an estimate, not a promise

Days of on-hand cover are calculated from current sellable stock divided by average daily sales. The stockout date is the local browser date plus the full days of projected cover.

Actual runout changes when demand varies, orders are cancelled, returns occur, stock records are wrong, inventory is reserved elsewhere, or inbound receipts arrive late.

Use a stockout estimate as a prompt for review, not as a guarantee of availability.

Stockout date = today + (on-hand units ÷ average daily sales)

Small example: 90 units on hand at 6 units per day ≈ 15 days of cover.

Common mistakes: counting reserved or unsellable stock as available; assuming constant sales through a season change; assuming inbound stock arrives in time without confirming the date.

Economic order quantity: a balancing point, not an answer

Economic order quantity (EOQ) estimates an order size that balances annual ordering cost against annual holding cost for a single product. The calculator on this site uses the classic square-root formula.

EOQ = √(2 × annual demand × cost per order ÷ annual holding cost per unit)

The formula assumes demand, ordering cost, and holding cost stay stable across the year. Real purchasing rarely behaves that way: quantity price breaks, seasonal demand, cash limits, and supplier rules all push the practical order size away from the mathematical optimum.

Treat EOQ as a reference point for comparison, not as an instruction to order that exact quantity.

Small example: annual demand 1,200 units, $25 to place an order, $3 per unit per year to hold: EOQ = √(2 × 1,200 × 25 ÷ 3) ≈ 141 units per order — roughly 8–9 orders a year.

Common mistakes: applying EOQ to seasonal or newly launched products with unstable demand; ignoring quantity price breaks that can beat the formula; treating EOQ as an exact instruction instead of a comparison point.

Landed cost: the real cost of one unit

Landed cost is the unit cost of a product once shipping, duties, insurance, and other shipment-level charges are included. The calculator on this site spreads shipment-level costs across the units in the shipment and adds them to the product cost.

Landed cost per unit = product cost + freight + duty + insurance + other fees, each per unit

Pricing or margin decisions based on the supplier invoice alone can overstate profit, because the excluded charges are real. Duty may be a percentage of value or a fixed amount, and freight terms decide which charges you actually pay.

A landed cost is per shipment: rates, exchange movements, and charge mix change between orders, so re-check the inputs each time.

Small example: product cost $10.00; freight $450 spread over 300 units ($1.50); duty 5% of product cost ($0.50); insurance $0.20 → landed cost ≈ $12.20 per unit.

Common mistakes: pricing from the supplier invoice alone; ignoring exchange-rate movement between order and payment; reusing one shipment’s landed cost for a later shipment with different rates and charges.

Inventory holding cost: what storage really costs

Holding cost is the annual cost of keeping inventory: capital tied up in stock, storage space, insurance, and shrinkage or obsolescence. The calculator accepts a single overall rate, or a build-up from those components, applied to average inventory value.

Annual holding cost = average inventory value × holding cost rate

Published rates vary widely by industry and product type because capital cost, storage cost, and spoilage risk differ. A rate copied from a general article may not describe your warehouse contract or product risk at all.

The most defensible rate is one built from your own costs: capital charge, storage, insurance, and expected losses.

Cash tied up in inventory: money you cannot spend

Inventory is cash in another form. This estimate values the stock you hold at unit cost and shows how long that cash stays locked up at your current sales rate.

Cash tied up = on-hand units × unit cost, released over days of cover

Recovering that cash depends on selling the stock at a margin after channel fees, shipping, and returns. Slow-moving or overbought inventory returns cash later than planned, or not in full.

Cost value is not guaranteed recovery value: discounts and fees reduce the cash you actually get back.

Small example: 400 units on hand at $8.50 cost = $3,400 tied up; at 9 units per day, that is about 44 days of cover before the cash converts back.

Common mistakes: valuing stock at retail price instead of cost; forgetting that channel fees and returns reduce recovery; overlooking slow-moving SKUs that lock cash indefinitely.

Inventory turnover: speed of conversion to sales

Inventory turnover compares the cost of goods sold in a period with average inventory value. Higher turns mean stock converts to sales faster; days of inventory on hand divide the period by the turns.

Turns = COGS ÷ average inventory value; days on hand = period days ÷ turns

Turnover differs sharply between categories, price points, and seasons, so a single universal target is meaningless. Compare a product against its own history and against comparable SKUs before judging the number.

A high turnover with frequent stockouts can be as costly as a low turnover with idle stock.

A simple peak-season workflow

One way to connect the tools before a busy season or promotion. The order is a review aid, not a policy: adjust it to how you actually buy.

1. Measure how fast each SKU sells. Inventory Turnover Calculator — separate fast movers from slow stock before committing cash.
2. Check what a bigger buy means for cash. Cash Tied Up in Inventory Calculator — see how much money is already on the shelf.
3. Set the buffer. Safety Stock Calculator — cover demand and lead-time variation for the season.
4. Set the trigger. Reorder Point Calculator — the stock level that starts the replenishment review.
5. Size the order. EOQ Calculator for a cost-balanced reference, then Purchase Order Quantity Calculator for MOQ and case-pack reality.
6. Know the true unit cost. Landed Cost Calculator — freight, duty, insurance, and fees per unit.
7. Put it together. Reorder Planner — one SKU-level view of timing, risk, and quantity.

Recalculate with current data as the season approaches; inputs from last quarter rarely describe this quarter.

Download the SKU worksheet (CSV)

Before using a result

Review the product’s current sellable stock, demand definition, supplier lead-time boundary, confirmed inbound timing, safety-stock policy, MOQ, case packs, cash constraints, storage capacity, shelf life, and supplier conditions. For unusual, seasonal, intermittent, or newly launched products, a simple average-based calculation may be less representative.

Use the Reorder Planner for a combined planning view.