Google Trends is the go-to free source for "which of these is more popular?". But it has two limits that break any serious keyword research: Max 5 terms per comparison. Each request is rescaled to its own maximum = 100. So if you export 20 batches of 5 keywords and paste them together, a "50" in batch 1 and a "50" in batch 7 are different things. In a simulation with known ground truth (Google-style rescaling + integer rounding), that naive concatenation was off by more than 20 ...