Stacks and slots
Most items and blocks stack to 64, so 1,000 cobblestone is 15 full stacks and 40 left over, filling 16 slots. Items that stack to 16 fill four times as many slots, and items that do not stack take a slot each. Pick the stack size, or pick the item from the list if you are not sure.
Containers
A shulker box, a single chest and a barrel each hold 27 slots; a double chest holds 54. Your inventory holds 36, the 27 main slots and the 9 in the hotbar; armor and offhand slots are not counted. A double chest full of shulker boxes, each full of a 64-stack item, is 54 × 27 × 64 = 93,312 items. The calculator shows how many of each you need, how many are full, and how many slots of the last one are taken.
Counting back
Switch to Stacks to items and put in what you have: double chests, single chests, shulker boxes, stacks and loose items. Every container is counted full, so it is the most it can hold. The total updates as you type, and the address keeps the numbers so you can share them.
Questions
Which items stack to 16?
Ender pearls, snowballs, eggs, signs and hanging signs, empty buckets, banners, armor stands, honey bottles and written books. Buckets of water, lava, milk or powder snow do not stack at all.
Which items do not stack?
Tools, weapons and armor, potions, enchanted books, totems, beds, boats and minecarts, shulker boxes (even empty ones), full buckets, cake, soups and stews, music discs and saddles.
Is it the same in Bedrock Edition?
For the items listed here, yes: they stack to the same sizes in Java and Bedrock, and the containers hold the same number of slots. Stack sizes changed with a command or data pack are not covered.
Does it save anything?
No. Nothing is stored and nothing is sent anywhere. The numbers are kept in the page's address, so a copied link opens the same count.