Topic difficulty is a 0–100 score of how hard it is to promote content for the keywords of a topic cluster in Google. Lower is easier. It is an average of the difficulty of everything inside the cluster - so it describes the subject as a whole, not its worst corner.
Where you see it
On each Topic cluster card, the score appears as Difficulty, next to Volume and Intent; In Sort by and in filters, the same score is called Topic cluster difficulty — one metric, two labels.
How it relates to Keyword difficulty
The score is built in two averaging steps, from the bottom up. Every keyword in your project has its own keyword difficulty [link is coming] - a score of how strong the results for that one phrase are. A page’s Page difficulty - the column you see on the Pages screen - is the average keyword difficulty of that page’s keywords. Topic difficulty is then the average Page difficulty across the cluster’s pages, rounded to the nearest whole number.
So the three difficulty scores in the app are one measurement at three zoom levels: keyword, page, cluster - each level the average of the one below it.
Why a cluster can be easier than its hardest keyword
First, arithmetically: because the score is an average, one brutal keyword is diluted by every ordinary one around it - it raises the score a little, it does not set it. And second, practically: you do not win a cluster by winning its hardest keyword. A cluster is covered page by page, and if most pages face ordinary competition while one faces giants, the sensible plan is to build the ordinary pages first - the cluster’s traffic is mostly there anyway.
The hardest keyword is one door into the subject; the cluster has many. Topic difficulty describes the typical resistance across all of them, not the worst case. A cluster with a moderate score and one very hard keyword is still, for practical purposes, a moderate cluster - you simply leave the hardest door for last, once the rest of the subject is covered and working in your favour.
How to read it
Compare clusters with it, at similar potential. When two clusters promise similar potential traffic, the lower-difficulty one is usually the faster path. When potential differs a lot, read them together - a hard cluster can still be worth it, but you should choose it knowing the price.
Read it next to Weak spots. Difficulty measures the average strength of the competition; Weak spots counts the individual openings in it. The pair disagreeing is normal and useful: a difficult subject that still shows weak spots has a way in; an average subject with none is more closed than its score suggests.
It describes the competition, not you. Difficulty is the same for every site looking at the subject. Whether your site can afford that competition is what Weak spots - which is relative to you - is for.
What next
- What is keyword difficulty? [link is coming]
- What are Weak spots?