5.5.4 Calculating the critical stock level (or ‘time to order’)

It is important to be aware of your vaccine stock, and to place your next order at the right time. The critical stock level (or ‘time to order’ level) is the number of vaccine doses in stock at the time when it is absolutely necessary to place a new order. ‘Time to order’ calculations take into account the level of vaccine consumption while waiting for the new supply. This precaution is necessary to prevent the vaccine stock from dropping below the minimum stock level before the new order can be collected. The delivery time is the time interval between the day the vaccines are ordered and the day that you collect them from the health centre.

The critical stock level can be calculated using the following equation:

  • scritical = qdelivery + smini , where:
  • scritical is the critical stock level
  • qdelivery is the number of doses needed during the delivery time, and
  • smini is the minimum stock level.
  • qdelivery = (qperiod x tdelivery) ÷ n , where:

The number of doses required during the delivery time (qdelivery) can be calculated using the following equation:

  • tdelivery is the number of days between placing the order and collecting new vaccines
  • n is the number of days in the supply period (i.e. the period that the health facility maintains its vaccine stocks at, or above, the minimum level).

Now attempt Activity 5.1 to calculate the critical stock level for an imaginary Health Post. You will need paper and a pen or pencil to help you make the calculations.

Activity 5.1  Calculating the critical stock level

At a particular Health Post, it often takes 2 working days for the HEW to collect her new supply of vaccines after ordering them from the health centre, which is 15 km away along difficult tracks. The number of doses of OPV she needs for a supply period of 2 weeks is 80 doses.

What is the critical stock level (or ‘time to order’ level) for OPV at this Health Post? Assume that the minimum stock level is 25% of the number of doses needed for the 2-week supply period (10 working days).

Write down all your calculations so that it is clear how you reached your answers. Do this before you check our answer below.


The calculation of the critical stock (or ‘time to order’) level can be done using the equations given above Activity 5.1. First, calculate the number of doses you will need during the delivery time (qdelivery) by using the equation:

  • qdelivery = (qperiod x tdelivery) ÷ n

In this Health Post, qperiod is 80 doses of OPV, tdelivery is 2 days, and the supply period is 10 working days (2 weeks) so n is 10.

  • qdelivery = (80 x 2) ÷ 10 = 16 doses

The minimum stock level (smini) is 25% of 80 doses, which is 20 doses. The critical stock level (scritical) is calculated using the equation:

  • scritical = qdelivery + smini

In this Health Post, qdelivery is 16 doses and smini is 20 doses, so:

  • scritical = (16 + 20) = 36 doses.

So the staff at this Health Post should place an order for OPV when the critical stock level is reached at 36 doses.

In the next study session you will learn about keeping vaccines and diluents safe and effective by maintaining the cold chain. This is vitally important in reducing wastage, as well as maintaining the potency of the vaccines.

5.5.3  Calculating the maximum stock level

Summary of Study Session 5