This is definitely going to be a debated topic as there are many furnace fuels in Minecraft. Let’s discuss the variety of fuels available, their smelting capabilities and why I think the best fuel is something you’re probably not using.
The typical progression of a game of Minecraft in terms of smelting starts with burning coal or charcoal. As you advance in the game depending on your resources or farms you likely move to Lava Buckets, Dried Kelp Blocks, Blaze Rods or Scaffolding. Let’s address these fuels in greater detail.
Scaffolding
First of all Scaffolding smelts 6 items on Bedrock edition only and that was only prior to the 1.19.40 update which released on October 25th, 2022. With the 1.19.40 update using Scaffolding as a furnace fuel in Bedrock edition has been completely nerfed as Scaffolding now only smelts 1/4th of an item now inline with Java edition. We can conclude this fuel is no longer a top contender as one of the best fuels.
Lava Bucket
Lava Buckets are certainly one of the best sources of fuel, they smelt 100 items per bucket. Of the many features added to Minecraft in the Caves and Cliffs update, one of them was lava became a renewable resource. This has made Lava Buckets one of the best fuels for smelting as you can set up a lava farm right in your smelting room and have quick access to nearly unlimited furnace fuel. Although Lava Buckets is one of the best furnace fuels, in most situations I will argue there is still an even better fuel.
Dried Kelp Block
Dried Kelp Blocks have long been a great source of furnace fuel as they smelt 20 items. Setting up a massive kelp farm is relatively easy and with some work and resources you can make an automated farm to collect tons of kelp. This has been one of the best furnace fuels in Minecraft for a long time. However now that lava is renewable I’d argue there is no point in using Dried Kelp Blocks as a long-term furnace fuel as lava is much easier to work with. Yes you do need buckets and then scoop up the lava but for Dried Kelp Blocks you have to smelt kelp, then manually craft the Dried Kelp into Dried Kelp Blocks.
In terms of efficiency if you are using Dried Kelp Blocks to smelt kelp you end up using almost half of the smelting power of one Dried Kelp Block (9 out of 20) to produce one more Dried Kelp Block. Lava Buckets are certainly now much better than Dried Kelp Blocks unless your goal is also to farm the experience from smelting kelp into Dried Kelp, then it might make sense to use Dried Kelp Blocks.
Blaze Rod
Blaze Rods are another great fuel source as they can smelt 12 items. They are especially great source of fuel if you have a Blaze farm setup. Blaze farms double as away to make tons of experience and you end up with lots of Blaze Rods. However, this is still not the best source of fuel in the game.
Coal and Charcoal
Coal and charcoal will smelt only 8 items but the practicality of using coal as a long-term fuel source is unlikely unless you have a Wither Skeleton farm and you’re getting renewable coal. Charcoal is renewable and easy to make however it requires growing and chopping down lots of trees which is very time consuming and if you burn charcoal to make charcoal you’re basically reducing your smelting efficiency to 7 items per charcoal as you have burned an entire charcoal in the process of making 8 charcoal. So coal and charcoal are certainly not the best furnace fuels.
The best furnace fuel
There are many other items in the game that can be used as furnace fuels from wooden tools, all the wooden block types and even saplings (yes saplings smelt 0.5 items per sapling). In Java edition even wool and carpet can be smelted for 0.5 and 0.335 items respectively. Of these other items that can be used as fuel one of them is actually the best source of fuel in the game. All of them except this one item tend not to be the best because they only smelt between 1.5 and 0.5 while also requiring too many resources to gather or craft.
The best source of fuel in Minecraft is the Wooden Slab. Wooden Slabs do only smelt 1.5 items, which is actually an additional benefit of using Wooden Slabs as a fuel source, I’ll explain that soon. First, let’s do some math, 1 log which only smelts 1.5 items crafts into 4 planks, each plank can smelt 1.5 items, so by crafting logs into planks you multiply the smelting power by 4 already. Then 3 planks crafts into 6 slabs and each slab can smelt 1.5 items. This is likely a smelting calculation error by Minecraft as a log which smelts 1.5 items can become 8 slabs which smelt 12 items in total. Looking at the easy abundant access of this resource and comparing it to it’s smelting power it is simply the best. For example, chopping one Dark Oak Tree which is one of the easiest tress to chop down will yield about a minimum of 30 logs, once those logs are crafted into to slabs you’ll have 240 slabs (30 logs x 4 planks = 120 planks, 120 planks x 2 slabs = 240 slabs). Those 30 logs become 240 slabs and can smelt 360 items, that’s almost over powered.
Now lets look back at our efficiency, many times when you smelt items you probably try to be efficient, such as gathering 8 items to smelt when you burn one coal or 20 items when you burn one Dried Kelp Block. If you’re not doing that you’re wasting tons of fuel. For example if you need to smelt 5 item and use a Lava Bucket you’re wasting 95 items worth of smelting power or if you used a Dried Kelp Block you waste 15 items worth of smelting power. When you use Wooden Slabs you only waste up to 0.5 items of smelting power if you smelt a quantity of items not divisible by 3. So if you have 5 items of something to smelt don’t wait until you collect 3 more or 15 more or 95 more, just toss them into your furnace filled with Wooden Slabs and forget about the 0.5 wasted fuel.
Personally I usually have a super smelter that’s at least partially automated for bulk smelting of chests full of items. For this I do recommend using Lava Buckets with a nearby lava farm. However, for all of my other furnaces around the world for one off items Wooden Slabs are easily by far the best. They are especially the best in early game, you don’t have to look for and mine coal. Simply craft 3 logs into 12 planks and 12 planks into 24 slabs and now you can smelt up to 36 items. You also don’t have to wait to collect groupings of 8 items to smelt to be efficient unlike using coal, now you can save your coal to sell to villagers or make torches.
Hopefully you find burning Wooden Slabs a useful tip, and of course happy crafting!
For the detailed list of furnace fuels and smelting details check out The Minecraft Wiki on Smelting.