Balancing

Tip

This assumes you want your mod to be balanced compared to the base game. If you don't, disregard it completely.

In any case, do lend an ear to player feedback: they are most of the users, and have collectively tested the mod more than you possibly can.

Lastly, perfect balance may be impossible for complicated cases: better aim for good enough.

Why balance

That should be common sense, but even novice modders can take 5 minutes to modify a base game weapon to become a free, easy to place, long range, auto-win machine. That's not interesting because it's not balanced.
If you use it against your friends they have no chance of winning and won't play against it again, and if you fight against it without it, you'll do the same.
Now nobody's playing anymore, which is bad. Alternatively, if everyone has it, all other weapons become irrelevant, and you've just narrowed down the game instead of adding to it, which is bad.

How

There are many ways to give some sort of balance to your mod, but the most logical is to use the base game as reference. Most notably, ratios (we'll come back to this below).

On the surface, that's very simple : Corridor has 1000 HP, 1x1 armor has 4000 HP. Note that we're already applying a good method by comparing parts with the same size.

So just make super armor in your mod and it give it 8000 HP, ultra armor has 16000 HP and so on, right? Not so fast.

Limits

First of all, it does not make sense for some numbers to go up (or down) infinitely, no matter how much you increase the cost of the part to justify it.

More powerful weapons IRL tend to be larger than less powerful weapons, because there is only so much power (gunpowder, amount of pellets, mass of uranium...) you can possibly fit into a given amount of space with a given technology. At some point even the technology used stops mattering. This is why guns are small, some cannons are really big, and black holes exist.

In short, power needs space (and mass where applicable), and too extreme numbers make no sense. Base game exceptions include flak "bullets" having infinite speed.

Logic

You can place a doorway that goes through stacked crew bunk beds like people are going to carry cargo while crawling in-between the beds, but that's nonsense. It pushes the player out of the immersion into the game, and they might go "Whoa this is so silly, clearly just a game, heck they don't even eat - hey did I forget the chicken in the oven?" and stop playing, which is bad.

Ratios

In reality, armor in general has 4000 HP per full tile, with a Density (increases your ship's mass and slows it down) of 3 for a cost of 8 steel.

The word "per" (or "for each" etc.) immediately means there is a ratio at play.

Corridor on the other hand, like most parts in the game, has a density of 1 for a cost of 4 steel.

Ratios tend to stay constant, which allows you to just apply them to a different part with a different stat to find out what the HP or steel cost (for example) of that part should be. That's very good.

These are very much the sort of thing you should check in a table :

Part HP/density and HP/steelCost Ratios

Part	Steel cost	Density	HP	HP per steel (ratio)	HP per mass (ratio)
Corridor	4	1	1000	250	1000
Armor	8	3	8000	1000	2666
Structure	2	0.33	1000* (doesn't block shots)	500	3000

Takeaways :

1. Heavy armor gives 4 times more HP per steel than non-armor parts. That's the ratio. If you decide one stat your part should have, this will give you the balanced amount for the other stats.
2. The bad HP/steel ratio for corridors is justified by the fact that they allow fast movement. If you make a part that both allows fast movement and is resistant, you should use an even worse ratio.
3. Structures are incredibly strong, but they can't protect you from shots, only intrusion and ship splitting. This is balanced as a specialized form of defense.

Tip

We have compared parts only using a single resource.
Rarer resources usually give better ratios or provide access to higher tech functionality. They also increase cost; that's particularly relevant in multiplayer mods that have a limit on that.

Non-numerical advantages

Anything that adds a beneficial feature counts for balance. One notable, common example is being roof-mounted, like the Deck Cannon.

This means adding a part copied from the base game except it's now roof-mounted, should be accompanied with nerfs because it's now a lot stronger if it keeps the same stats.
This is if it even makes sense in the first place : maybe a roof-mounted thruster should push your ship down, not forward? You decide and should balance accordingly.

Also, roof-mounted usually implies that the part blocks shots form roof-mounted weapons, and cannot be placed near a Deck Cannon. The same limitations should apply to your added part(s).

See this part of Data Fields for a how-to.

For shields, the same can be said about projecting through parts, like the small shield does.

Easily make balanced mods

Here are some types of content you can make with mods that would generally guarantee that it's balanced.

Series extensions

Heavy armor comes in many shapes and sizes in the base game. So do Laser blasters and Cannons, but that's trickier because many of their stats change.

Heavy armor has some data fields that remain the same no matter what shape we're talking about : Density being the most important here.

If your mod adds shapes of Heavy armor with the same Density and respects the health/mass/cost ratios, you are simply extending the series of available heavy armor shapes.

Your mod will automatically be balanced as a result.

Example mod : "More crew quarters".

Variants

If you add a part that is an exact copy of another part except for 2 stats, one increased by X% and the other decreased by the same X%, it's probably balanced. A few considerations though :

1. The increased/decreased stats should both be beneficial or both be detrimental, else it's just a double-buff (overpowered) or double-nerf (too weak). Example mod : Radar sensors.
2. Logic (see section above) still applies. A part with 100% reduced mass makes no sense regardless of other stats, because matter has mass.
3. If you choose to increase a beneficial stat and the cost, consider the ratio of power per tile used. More power should demand more space. Conversely, reducing power should allow you to offer a smaller variant. Example mod : Dogfighters.

Combining ratios

It is balanced to make a part that has no equivalent in the base game, if it respects multiple ratios for its size, cost, HP, and takes a little away from them to add new non-numerical advantages.

Example mod : Bulkheads.