The takeaway
Iron Mountain Incorporated shows a moderate seasonal pattern over 10 years of data — strongest in August (+3.5%) and softest in September (−2.2%).
Right now
In July, the stock has risen 70% of years, averaging +3.1%, about +1.0 pts better than the S&P 500.
The full picture
Iron Mountain Incorporated's most dependable month has been August, higher in 8 of 10 years; September has been its least reliable, up just 40% of the time.
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Month by month
The stock's clearest edge over the S&P 500 lands in January (+3.8 pts); it has trailed the market most in September (−2.0 pts).
“vs S&P” is Iron Mountain Incorporated’s average for a month minus the S&P 500’s average for that same month — isolating Iron Mountain Incorporated’s own seasonal edge from broad market drift.
Reality check
Over the last 5 years, August has closed higher 80% of the time versus 80% across the last 10 years — the pattern is holding.
Figures are the typical (median) August return and how often it rose — the last 5 years versus the last 10(the heatmap’s default window). This verdict stays anchored to that 10-year window even if you zoom the chart, so it never disagrees with the badges above.
In plain English
This is a stock you can almost set a calendar by, and August is the anchor — it has closed higher in 8 of 10 Augusts, the steadiest beat on its year.
The strength looks broad-based rather than freakish: its average (+3.5%) and median (+3.4%) sit close together, so no single blow-out year is flattering the figure. No month is steadier: August's returns vary by just 4.5% year to year. Better still, that strength is the stock's own and not just a buoyant market — August has outpaced the S&P 500 by +3.1 points on average. Few peers keep such company in August — the typical stock clears it just 52% of the time.
August anchors a run, too: the March-through-August window has been the stock's reliable season. On the other side of the ledger, September has been the soft spot — the only month to average an outright loss (−2.2%), and the edge isn't year-round — the stock has trailed the S&P 500 in September, November, and October. Its roughest month on record was a −23.1% March in 2020 — a reminder of how hard even a seasonal name can fall.
One run worth flagging just ended: a 8-year streak of positive Augusts was snapped by a −1.9% close in 2025. Reassuringly, the tendency has held its shape: the recent five years still track the years behind them.
For a stock this dependable in August, the sharper question is the rest of the year — outside its strong stretch, the calendar gives far less to lean on.
Short answers on the stock's best month (August), its worst (September), and whether it really trades seasonally.
Yes, to a moderate degree. Since 2016 its best month (August, +3.5%) has run well ahead of its worst (September, −2.2%) — the heatmap above shows how steady that gap has been year to year.
August has been the strongest, averaging +3.5% and closing higher in 8 of 10 years since 2016.
It's the weakest, averaging −2.2% — historically a soft spot, though it still varies from year to year.
Explore
These names have the strongest July track records on record — a starting point for comparison.
Before you trade