I am not familiar with the MISP calculator sheet for SRH. I guess you want some way of using demographic data (e.g. from census reports or DHS/MICS survey reports or data that should be known to SRH programs) to estimate the number of lactating women in the population. I think the easiest way to do this is start with population and birthrate. If these are:

  Population (both sexes) : 110,000
  Birthrate : 44 / 10,000 population
    
then the number of births per year can be estimated:
    
  Births per year = Birthrate * Populations
  Births per year = 44/1000 * 110000 = 4840
    
We might assume that all new mothers lactate until breastfeeding ceases. We can use IYCF data (also from DHS or MICS) to estimate the average duration of breastfeeding. If each new mother breastfeeds for an average of 18 months (1.5 years). Then we'd expect about:

    4840 * 1.5 = 7260

breastfeeding women to be present at any one time. Does this make sense? It might be useful starting point. 

This is a crude estimator as not all children survive for 1.5 years (mothers may also die), population estimates can be way off in emergencies and population may change quickly in emergencies, and birth rates may change over time.

You might be better off making estimates directly from IYCF surveys and apply proportions to population numbers.

I hope this is of some use.

Dear Sarah:

Mark's response is great, but why do you want to know the number of lactating women in a population?  Such women are a very heterogenous group. A woman who is practicing exclusive breastfeeding of a big 6-month old infant experiences a much greater demand on her metabolism, nutritents, time, etc. than a women providing occasional comfort breastfeeding to a 3-year old toddler. Moreover, the risks to the child from not breastfeeding would be very different in these 2 children.  In our nutrition surveys, we sometimes separately analyze lactating women, but often include in such an analysis only those women breastfeeding a child less than a certain age, often 6 months, in order to include in the analysis only those women with the greatest metabolic demand.  Estimating the number of such women would be a bit more difficult and require additional data from IYCF assessments.

Dear Sarah,

In cases where it is not possible to get population estimates of the different target groups a 9% average is usually used for pregrant and lactating mothers where 5% is for pregrant and 4% for Lactating women.

Woody makes a good point. For program planning, I think you will be most interested in the subset of women breastfeeding a young child. The standard IYCF group of interest is 0-23 months but you may want to restrict this to younger children such as <= 12 months or < 6 months (as Woody suggests).