Inventory how-to

How to plan safety stock for your top sellers

Safety stock is extra inventory held to cover demand spikes or supplier delays during lead time.

Short answer: Safety stock is extra inventory held to cover demand spikes or supplier delays during lead time. For top sellers, set safety days based on how unpredictable their demand and supplier lead times are, then convert that into units using recent daily sales. Review it every few months, since a product's sales rate changes as it grows or ages.

Why top sellers need the most attention

Your best sellers cause the most damage when they stock out. A slow mover running out costs you a few sales. A top seller running out can cost weeks of revenue and push customers to buy the item elsewhere. These are the SKUs where getting safety stock right matters most.

How to size safety days

Start with two questions: how much does demand swing week to week, and how reliable is the supplier's lead time. If both are steady, a small safety buffer (5 to 7 days) is enough. If demand is choppy, or the supplier sometimes ships late, push it higher (10 to 15 days or more). You don't need a formula for this. Look at your last few months of weekly sales for the SKU and your last few purchase orders to that supplier, and judge how much they vary.

Convert days into units

Once you have a safety day number, multiply it by the product's average daily sales to get a unit buffer. Add this to your lead-time demand to get your full reorder point. Recalculate daily sales regularly, since a top seller's run rate shifts with trends and seasons, and a safety stock number based on old sales data gets stale fast.

Keep it under review

Safety stock is not a one-time setting. Revisit it after big sales events, supplier changes, or when you notice a SKU trending up or down. If you are stocking out between orders, your safety days are too low. If you are sitting on excess for months, they are too high.

Step by step

  1. List your top 10 to 20 SKUs by recent revenue or units sold
  2. For each one, pull the last 3 to 6 months of weekly sales and note how much they vary
  3. Check each supplier's lead time and how consistent it has actually been, not just what they quote
  4. Assign safety days per SKU: low for stable demand and reliable suppliers, higher for volatile ones
  5. Multiply safety days by average daily sales to get the unit buffer
  6. Add that to lead-time demand to set the reorder point per SKU
  7. Recheck every 2 to 3 months or after any stockout or big sales spike

Worked example

Say your top seller sells 20 units a day on average, with sales ranging from 14 to 30 depending on the week. Your supplier's lead time is 10 days and has occasionally run 2 to 3 days late. You set safety days at 8 to cover both the demand swing and the supplier's slack. Reorder point = 20 x (10 + 8) = 360 units. If you currently have 150 on hand and 0 on order, and you want 14 days of cover beyond the reorder point, order quantity = 20 x (10 + 8 + 14) - 150 = 490 units, rounded up to the nearest case pack your supplier ships.

How SteadyShelf helps

SteadyShelf lets you set safety days per SKU per supplier and automatically rolls them into the reorder point and suggested order quantity, so you don't have to recalculate this by hand each week. It does not model demand volatility for you or flag which SKUs need wider safety margins, so the judgment call on how many safety days to assign is still yours.

$29/month flat, unlimited products and suppliers, 30-day free trial. Run the numbers now with the free reorder point calculator.

SteadyShelf is launching on the Shopify App Store. Join early access to get the install link first.

Related guides

All inventory how-to guides ยท Inventory glossary

FAQ

Should every product get the same safety stock buffer

No. A flat buffer wastes cash on stable, predictable SKUs and still leaves you short on volatile ones. Set safety days per SKU based on its own demand variability and its supplier's lead time reliability, not a single store-wide rule.

What if I don't have enough sales history for a new top seller

Use whatever weeks of data you have and lean toward a higher safety buffer until the pattern is clearer. Once you have a few months of consistent sales, tighten the buffer to match actual demand swings rather than guesswork.

Let SteadyShelf do this math for every SKU

Order-by dates per supplier, seasonal forecasts and draft POs for your Shopify store. $29/month after a 30-day free trial.

Get early access