What the Inventory Reorder Point Calculator does
The reorder point is the stock level at which you place the next order so that it arrives before you run out. This calculator works it out as the demand you expect during the supplier's lead time plus a safety stock, and shows every step with its units - so a weekly sales figure and a lead time in days cannot be quietly multiplied together.
You can type a safety stock you already use, or let the page calculate one from a service level and the variability of demand and lead time. If you enter your current stock position, it also tells you whether to order now or how many days you have left.
How to use it
- Choose whether demand is per day or per week, and whether your weeks have 7, 6 or 5 days. Lead time must be counted in the same kind of day: if the shop is shut on Sundays and demand is per working day, count lead time in working days.
- Enter average demand and the average lead time from order to goods being available to sell - including receiving and put-away, not just the carrier's transit time.
- Set safety stock: either type the buffer you use, or choose From service level and enter the standard deviations of demand and lead time and a cycle service level.
- Optionally enter your inventory position: stock on hand plus stock already on order, minus back orders. The page says whether you are at or below the reorder point.
- Copy the step-by-step working or download it as CSV to set the reorder level in your stock system.
Reading the results
The headline is rounded up to a whole unit. Set that figure as the reorder level (sometimes called the min or order trigger) in your inventory system; when the inventory position drops to it, raise a purchase order.
Lead-time demand is what you expect to sell while waiting for the delivery. Safety stock is what protects you when demand runs above average or the delivery is late. If the bars show safety stock bigger than lead-time demand, variability dominates and better forecasting or a more reliable supplier would save more than a bigger buffer.
Compare against inventory position, not physical stock. Units already on order will arrive before the next cycle, so ignoring them makes you order twice.
Worked example: a cafe's takeaway cups
A cafe uses 40 packs of cups a day with a standard deviation of 8 packs. The wholesaler delivers in 5 days on average, give or take 1 day, and the owner wants a 95% chance of not running out between order and delivery (Z = 1.644854).
Lead-time demand is 40 x 5 = 200 packs. Safety stock by the combined method is 1.644854 x sqrt(5 x 8^2 + 40^2 x 1^2) = 1.644854 x sqrt(320 + 1,600) = 1.644854 x 43.818 = 72.07 packs.
The reorder point is 200 + 72.07 = 272.07, rounded up to 273 packs, which is about 6.8 days of average use. The cafe has 420 packs on hand and on order, so it can wait about (420 - 272.07) / 40 = 3.7 days before ordering.
Formulas and scoring rules
- Reorder point
ROP = d x L + SSd in units per day, L in days, SS in units. The headline rounds up to the next whole unit.- Lead-time demand
LTD = d x L- Safety stock from service level
SS = Z x sqrt(L x sigma_d^2 + d^2 x sigma_L^2), Z = invNorm(service level)Demand and lead-time variability combined, assumed independent. 95% -> Z = 1.6449.- Weekly to daily
d = d_week / k; sigma_d = sigma_week / sqrt(k); L_days = L_weeks x kk = days in your week (7, 6 or 5).- Days of cover
cover = ROP / d; days until reorder = (position - ROP) / dposition = on hand + on order - back orders.
Continuous review, not calendar ordering
A reorder point assumes someone - or the stock system - checks the inventory position often, ideally after every sale or at least daily. If you only count stock and order once a fortnight, you are running a periodic review system, and the protection must cover the review period as well as the lead time: use L + R instead of L, where R is the gap between reviews.
The reorder point also says nothing about how much to order. Pair it with an order quantity, such as the economic order quantity, so that each order is large enough to be worth placing and small enough not to tie up cash.
Limitations: what the result does not prove
- It uses averages. If demand is seasonal or trending, recalculate the reorder point as the average changes; a static figure set in January will be wrong in December.
- The service-level safety stock assumes demand and lead time vary roughly normally and independently. Items that sell in occasional large lumps need a different approach.
- It does not know about minimum order quantities, pack sizes, shelf life or supplier holidays, all of which can justify ordering earlier or in different amounts.
- A reorder point prevents stock-outs only if the inventory records are accurate. Shrinkage and miscounts make the system believe stock exists that does not.
Privacy: where your data goes
Everything you paste, type or drop is processed in this browser tab. It is not uploaded, logged, stored or sent to analytics. Session recording and tag-manager scripts are switched off on this page.
Standards and sources
- NIST/SEMATECH e-Handbook of Statistical Methods - checked 19 Sep 2026
- NIST/SEMATECH e-Handbook of Statistical Methods - Normal distribution
Frequently asked questions
What is the reorder point formula?
Reorder point = average daily demand x lead time in days + safety stock. For 40 units a day, a 5-day lead time and 72 units of safety stock, that is 200 + 72 = 272 units. The units must match: demand per day with lead time in days.
Should I compare the reorder point with stock on hand or with inventory position?
With inventory position: stock on hand plus what is already on order, minus customer back orders. Using on-hand stock alone ignores deliveries already coming, and you end up placing a second order for the same shortfall.
What if my supplier quotes lead time in weeks but I track sales per day?
Convert one of them first. Choose weeks for lead time and set the number of days in your week; the page multiplies weeks by that number so both sides are in days, and shows the conversion in the working.
Can the reorder point be lower than the safety stock?
No. The reorder point is lead-time demand plus safety stock, so it is always at least the safety stock. If demand is zero the two are equal, which is a hint that the item may not need a reorder point at all.
How often should I recalculate my reorder points?
Whenever demand or lead time changes meaningfully, and at least each season for items with seasonal demand. Many teams recalculate monthly from the last 13 or 26 weeks of history so that the figures follow the business rather than lag it.
Last reviewed by the A2Z.Tools team against the sources listed above.
