Here are all the Undertale food and healing items and how much they heal.

All Healing Items (How Much They Heal & How to Get Them)

Ruins

• Monster Candy – 10 HP
• Spider Donut – 12 HP
• Spider Cider – 25 HP
• Butterscotch Pie – FULL HP
• Snail Pie – FULL -1 HP

Snowdin

• Snowman Piece – 45 HP
• Nice Cream – 15 HP
• Bisicle – 11 HP
• Unisicle – 11 HP
• Cinnamon Bunny – 22 HP

Waterfall

• Astronaut Food – 21 HP
• Crab Apple – 18 HP
• Sea Tea – 10 HP
• Abandonded Quiche – 34 HP
• Temmie Flakes – 2 hp
• Dog Salad – Random (2/10/30/Full)

Hotlands

• Instant Noodles – 4 HP (in battle) 15 (overworld) Full (serious mode)
• Hot Dog…? – 20 HP
• Hot Cat – 21 HP
• Junk Food – 17 HP
• Hush Puppy – 65 HP
• Starfait – 14 HP
• Glamburger – 27 HP
• Legendary Hero – 40 HP
• Face Steak – 60 HP
• Popato Chisps – 13 HP

Endgame

• Bad Memory – -1 HP (Full if less than 3 HP)
• Last Dream – 17 HP