Effective stock replenishment keeps inventory aligned with customer demand while minimising storage costs and lost sales. For growing UK ecommerce brands, this means calculating reorder points and quantities based on actual sales velocity and supplier lead times rather than guesswork.
This guide explains the core calculations, provides step-by-step methods to apply them, and outlines how to adjust for seasonality, supplier reliability and cash flow constraints so you can maintain healthy stock levels as your brand scales.
Understanding sales velocity
Sales velocity is the average number of units sold per day. It forms the foundation of every replenishment decision. To calculate it, divide the total units sold over a recent period by the number of days in that period. Use at least 30 days of data, and separate calculations by product or product group because velocities differ widely.
For example, if a product sold 180 units over 45 days, the daily velocity is 4 units. Track this figure weekly. Sudden changes in velocity often signal the need to investigate marketing campaigns, competitor activity or seasonal trends before adjusting stock orders.
Calculating lead time demand
Lead time is the number of days between placing a purchase order with your supplier and receiving the stock into your fulfilment centre. Lead time demand is simply daily sales velocity multiplied by the number of days in the lead time.
If your supplier takes 14 days to deliver and your velocity is 5 units per day, you will sell 70 units while waiting for the new stock to arrive. This figure tells you the minimum quantity that must be on hand when you place an order. Always add a safety buffer if your supplier has been unreliable or if demand fluctuates.
Setting the reorder point
The reorder point is the stock level at which you should place the next order. It equals lead time demand plus safety stock. Safety stock protects against unexpected spikes in sales or delays from the supplier. Conversion rates and buffer levels vary widely; benchmark against your own data although you can adjust this as you gather more data on variability.
Using the earlier example: lead time demand of 70 units plus 35 units of safety stock gives a reorder point of 105 units. When your inventory reaches 105 units, it is time to order. Many fulfilment platforms and inventory tools can alert you automatically once this threshold is crossed.
Deciding how much to reorder
Order quantity balances the cost of placing orders against the cost of holding stock. A practical approach for small brands is to order enough to cover expected sales for the next lead time period plus the following one. This avoids frequent small orders that increase shipping fees from suppliers.
If your velocity is 5 units per day and the combined lead time plus review period is 28 days, you might order 140 units. Compare this with any minimum order quantities set by the supplier. If the supplier requires 200 units, calculate whether the extra holding cost is justified by the discount offered or whether you should seek an alternative supplier.
Review the quantity every month. As velocity increases, increase order sizes proportionally while watching cash tied up in inventory.
Adjusting for seasonality and growth
Many UK ecommerce brands experience clear seasonal peaks, particularly in the final quarter. Calculate separate velocities for peak and off-peak periods rather than using an annual average. For example, a product that sells 3 units daily in summer but 12 units daily in November needs two different reorder points.
When your brand is growing, use a rolling 30-day velocity and update calculations every two weeks. If sales are increasing by 10-15% month on month, build this trend into your lead time demand forecast instead of relying solely on historical averages. Keep a simple spreadsheet with columns for date, units sold, velocity, lead time and reorder point so you can see trends at a glance.
Practical steps to implement replenishment
- Choose 5-10 of your fastest-moving products and calculate their current daily velocity using the past 30-60 days of sales data.
- Ask each supplier for their current average lead time in working days and note any minimum order quantities or shipping costs.
- Decide on a safety stock policy. Conversion rates and buffer levels vary widely; benchmark against your own data and adjust after two or three ordering cycles based on whether you experienced stockouts or excess inventory.
- Set reorder points and quantities in your inventory or fulfilment system. Test the settings with a small order first.
- Schedule a monthly review meeting or calendar reminder to recalculate velocities and update reorder points before cash flow or storage space becomes an issue.
- Keep records of every order placed, actual lead time experienced and any stockouts. Use this data to refine safety stock levels over time.
These steps can be managed in a spreadsheet initially. As order volume grows, consider connecting your sales channels to a dedicated inventory management tool that updates velocities automatically.
Common pitfalls and how to avoid them
Ordering too late is the most frequent mistake. Relying on visual checks or gut feel often results in stockouts during busy periods. Using the calculated reorder point removes emotion from the decision.
Over-ordering ties up cash that could be used for marketing or product development. Limit initial orders to no more than 8-10 weeks of supply unless the supplier offers a substantial discount that improves overall margin.
Another error is treating all products the same. Fast-moving lines need tighter monitoring and more frequent ordering, while slow movers can have higher safety stock but lower reorder frequency. Segment your catalogue by velocity to allocate time and working capital effectively.
Finally, failing to communicate with suppliers leads to longer or inconsistent lead times. Share your forecast for the next 90 days and confirm any planned promotions that will increase demand.
Frequently asked questions
How often should I recalculate my reorder points?
Recalculate reorder points at least once a month or whenever you notice a sustained change in sales velocity. Conversion rates and review frequencies vary widely; benchmark against your own data to keep calculations aligned with current demand.
What is a good safety stock level for UK ecommerce?
Conversion rates and buffer levels vary widely; benchmark against your own data. Increase this buffer if your supplier lead times vary by more than three days or if sales fluctuate sharply. Reduce it once you have several months of reliable data showing low variability.
Should I order more when my supplier offers a discount?
Only order extra if the reduced unit cost outweighs the additional storage and cash-flow impact. Calculate the total cost of the larger order including any storage fees at your fulfilment centre. Many brands limit extra stock to no more than an additional four weeks of supply.
How do I handle products with very different sales velocities?
Group products into fast, medium and slow movers. Set shorter review cycles and lower safety stock percentages for fast movers. Slow movers can tolerate higher safety stock and less frequent ordering. This segmentation prevents overstocking low-demand items while protecting high-velocity lines.
Can I manage replenishment without specialist software?
Yes. A regularly updated spreadsheet with daily sales, velocity, lead time and reorder point columns works well for brands selling fewer than 50 products. Automate data import from your sales channels where possible to reduce manual errors.
Related reading: Finding affordable fulfilment centres near West Yorkshire, How to add shipping tracking to your ecommerce store easily?, Managing seasonal inventory spikes for UK ecommerce sellers. Browse all fulfilment & logistics guides or apply to Loomworks.