Raw kale leads raw collards on calcium per 100 calories by 0.71mg — 725.71 against 725.00, a gap of 0.099%. That is not a win. It is a rounding artefact, and we are not going to pretend otherwise. USDA FoodData Central publishes kale at 254mg of calcium against 35 calories per 100g, and collards at 232mg against 32. Those four whole numbers are the entire basis of the ranking.
Every ranking post has a first place. This one does not, and the reason is worth more than the answer would have been.
Why 0.71mg is smaller than the error bar
The source figures are whole numbers. Kale is published as 35 calories per 100g, not 35.0 or 34.8 — so the true value sits somewhere between 34.5 and 35.5, and everything computed from it inherits that spread. Run both foods through their own rounding intervals:
| Food (USDA description) | Calcium /100 kcal | Range the source allows | Source row |
|---|---|---|---|
| Kale, raw | 725.71mg | 714.1 – 737.7mg | 254mg / 35 kcal |
| Collards, raw | 725.00mg | 712.3 – 738.1mg | 232mg / 32 kcal |
The two ranges overlap across essentially their whole length, and the 0.71mg gap is about 3% of the width of either one. Nudge kale's calorie figure to 35.4 and it drops to 717.5. Nudge collards' to 31.6 and it climbs to 734.2. Both nudges are inside what the published rows permit, and together they reverse the order. The ordering is an artefact of where two numbers happened to round.
The rest of the ranking is not close at all
This matters because the tie is specific to the top two. Third place is a long way back.
| Food (USDA description) | Calcium /100 kcal | Calcium /100g | Calories /100g |
|---|---|---|---|
| Kale, raw | 725.71mg | 254mg | 35 |
| Collards, raw | 725.00mg | 232mg | 32 |
| Arugula, raw | 640.00mg | 160mg | 25 |
| Cabbage, bok choy, raw | 476.15mg | 61.9mg | 13 |
| Spinach, raw | 430.43mg | 99mg | 23 |
| Seaweed, wakame, raw | 333.33mg | 150mg | 45 |
| Celery, raw | 330.71mg | 46.3mg | 14 |
| Chard, swiss, raw | 268.42mg | 51mg | 19 |
Raw arugula is 85mg behind second place — 11.7% — which is a real gap, more than a hundred times the size of the one separating the leaders. So "kale and collards lead the vegetables on calcium per calorie" is a defensible sentence. "Kale is the highest-calcium vegetable per calorie" is not, and it is the sentence every leaderboard would have printed.
Change the basis and the tie disappears
Per 100g, raw kale leads raw collards 254mg to 232mg — 9.5% clear, which is a genuine difference rather than a rounding wobble. Same two foods, same two rows, and one basis produces a real ordering while the other produces a coin flip.
The reason is that the per-calorie figure divides by a small number that was rounded. Kale's 35 calories carries a possible error of about 1.4%; collards' 32 carries about 1.6%. Dividing by those inflates the uncertainty in the answer to roughly 3% each way — enough to swallow a 0.099% gap whole. The per-100g figures divide by nothing and keep their precision.
The rule this hands you: when two foods finish within about 1% of each other on a per-calorie ranking, check whether the calorie figures are whole numbers before you believe the order. On this dataset they always are.
What we are actually claiming
Across the 37 vegetables we hold, raw kale and raw collards are jointly the most calcium-dense per calorie, at roughly 725mg per 100 calories each, and the data cannot separate them. Per 100g, kale leads. Pick either; the difference between them is smaller than the difference between two bunches of the same vegetable.
A caveat these rows cannot resolve: both are raw, and both are foods people almost always cook. Cooking changes water content and therefore both columns, and we hold no cooked row for either — so no wilted figure is given here rather than estimated. Calcium absorption also differs between foods, and nothing in a USDA composition row speaks to it.
The bit that survives contact with a real week
None of this changes what you should put in a pan. Kale or collards, whichever the shop had, cooked in something you like — the arithmetic was never going to decide that.
Where it does bite is the running total: 100 calories of raw kale is 286g, which is most of a bag, and it is very hard to guess what you actually ate. Sunn handles that inside WhatsApp — photograph the pan, or write "big pile of kale with two eggs", and the calcium and the calories come back in the thread. No app, no lookup, no deciding which of four kale entries matches your dinner.
More on this around the site: the full calcium leaderboard across all 189 foods, the all-foods calcium-per-calorie ranking this drills into, arugula against spinach — fourth and fifth on the list above — and why per-calorie rankings favour leafy greens in the first place.
Written by Espen Opdahl
Founder of Sunn. Building AI-powered nutrition coaching to make healthy eating simple. Nutrition data sourced from the USDA FoodData Central database.
Medical disclaimer: This content is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making changes to your diet or exercise routine.