KD stands for "Keyword Difficulty" and represents an estimation of how hard it would be to rank on the 1st page of Google (aka the Top 10 search results) for a given keyword.
It is a metric found on any report referring to Organic keywords in Site Explorer, Keywords Explorer, and Rank Tracker:
KD is measured on a scale from 0 to 100, with the latter being the hardest to rank for.
KD is calculated using our own unique algorithm, and it is based on the number of referring domains (RDs) the Top 10 ranking pages (organic search results only) for a given keyword have.
Note: KD does not take into consideration any "on-page SEO" factors.
How accurate is the Keyword Difficulty score?
Given Ahrefs' regularly updated database of backlinks and indexed pages, the KD score represents a reliably accurate picture of how competitive a SERP is, with regards to backlinks required in order to rank.
However we do not recommend you base all your SEO decisions on our KD score alone. Our Beginner's guide to Keyword research and our blog on How to choose keywords for SEO provides more in-depth and appropriate approaches to using the KD score.
KD can be considered a "first filter" to remove keywords that are too competitive, or as a way to prioritize keywords to target.
How can I see the KD scores a keyword used to have?
Next to the KD score you see a graph that shows the average keyword difficulty recorded for a keyword over time. Hovering over the graph shows a particular month's average keyword. You can export this data from the 3 dot menu at the top:
Why are some of the KDs in my report gray/black?
These are actually cached KD scores.
We regularly refresh SERPs for millions of keywords and calculate the KD score based on the pages we see ranking there at that time. KD that is grey means that the SERP has not been updated for 20 days or more.
Note: These cached KDs may not be the most accurate, but are still handy when you need to filter a large list with thousands of keywords.
If you want to refresh the SERP on a particular keyword, you can either click on Update on the top of the keyword's Overview page:
Or click the Refresh icon on the right most side of the table for other reports in Keywords Explorer. This allows you to quickly update SERPs for multiple keywords at once:
Note: The number of times you can refresh a keyword depends on the number of SERP updates per month you have left.
How often is KD refreshed for a keyword?
KD is recalculated every time SERPs are refreshed for a keyword in Keywords Explorer. This is done automatically for every keyword with significant enough of an estimated monthly search volume. Keywords with higher search volume will get SERPs updated for it more frequently.
For example the keyword rice has a estimated monthly search volume of 148k in the United States, and has its SERPs updated almost every day:
This is compared to the keyword healthy rice krispies which has a estimated monthly search volume of 90 in the United States, and has its SERPs updated about once a month:
Note: There's no formula to guarantee what day, or how often SERPs will automatically get updated for a keyword. But you can check the previous dates of which SERPs have been updated using SERP overview, to get a rough idea.