Convert string to integer in Shopify Liquid?
Using assign with a math filter is correct. See this thread on GitHub, and this blog post.
Variables created through {% capture %} are strings. When using assign, either of these options should give you a number:
{% assign var1 = var1 | plus: 0 %}
{% assign var2 = var2 | times: 1 %}
If this doesn't work for you, can you post the relevant code?