# calculate the population inside a buffer with census data

I have population data matched with polygons and a set of points that I have drawn buffers around. Assuming that the population within each geographical area, i.e. the polygons, is evenly distributed, how do I calculate the population within each buffer in ArcGIS? Some of the buffers are overlapping.

If you need I could send you the data.

• This is sometimes called areal allocation. I have a description of doing it in another answer (I'm sure there are others floating around this site as well.) Mar 5, 2015 at 18:28
• It's also known as apportionment, and this specific method is apportion by area. I discuss some of the implications and link to discussions on alternative methods in my answer at gis.stackexchange.com/questions/104219 Also related, for QGIS (but only in the tool used to summarize), is gis.stackexchange.com/questions/133832 Mar 6, 2015 at 2:05
• Reminder that since your circles overlap, you can't just sum the population within your circles to get 'people impacted'. If you need a number like that, you have dissolve the circles into polygons before you do your clip.
– Mox
Aug 17, 2022 at 22:29

This can be accomplished with an `Intersect`, followed by a `Field Calculate`, and then finally a `Summary Statistics`. Make sure that your buffer feature class has a unique ID field.

Before getting started, you will need to add a field (name `Polygon_Areas`, type `Double`) to your population polygon feature class, and then field calculate it, using `Shape_Area` as the input. This will create a field with your areas that can be carried over in geoprocessing.

Now perform an `intersect` with your population polygons and your buffers as inputs.

A new feature class is created. Add a new field to it - maybe `Area_Pop`, type `Double`.

Field Calculate this new field in your intersect feature class:

`Population field * (Shape_Area / Polygon_Areas)`

Finally, you can perform a summary statistics on your intersect feature class. Use the `Area_Pop` as your statistics field and chose `SUM` as your type. Add the unique ID field for your buffer feature class as your Case field.

Voila. You can perform a Tabular Join from your new table back to your original buffer feature class and add the `SUM_Area_Pop` field to a new field with a field calculate.

Good luck!

• if the population layer is raster, do you know how to do it ?
– toby
Mar 5, 2015 at 19:49
• You're welcome. I'm a vector guy; raster's not my bag I'm afraid. Mar 5, 2015 at 19:51
• Without running through a test I'm unsure - does this method (using Intersect) account for the overlapping buffers? Pretty sure it does (it'd just create two polygons for the same area covered by two different buffers if I remember right), but wanted to check. @Toby - raster should be pretty easy if you have Spatial Analyst. It'd be the Zonal Statistics tool with a bit of math/field calculation. But that would be best asked as a separate question. Mar 6, 2015 at 2:12
• Hi @ChrisW I had figure out that the "intersect tool" could solve the problem, there are some wrong with my layer at first, so I couldn't calculate the area size. by the way, I have the spatial analyst tools, could I only use the zonal statistics as table tool to get my results? thank you very much
– toby
Mar 6, 2015 at 3:24
• @toby It wouldn't be just zonal statistics - similar steps as here are necessary. It's just you'd be able to do it on raster rather than vector data. But again, that's best asked/researched as a separate question. Mar 6, 2015 at 3:59