Balance is a comparison of weights

Two ways to balance a page

Symmetry balances by mirroring. Put the same thing at the same distance on both sides of an axis and the result is stable, formal and slightly static. This is the right answer more often than modern design writing admits: certificates, invitations, title pages, official notices, anything where the message is this is serious and this is settled.

Asymmetric balance balances by weight. A large pale element near the axis can balance a small dark element far from it, in the same way a light weight on a long lever arm balances a heavy one on a short arm. The result is more dynamic and much harder to get right, because you are judging a product of two quantities rather than matching one.

The lever analogy is close enough to be useful. Weight, from the first block, is roughly area times darkness times detail. Distance from the axis multiplies it. So:

left side:  a 240px pale photograph, mean lightness 85%, 120px from centre
right side: a 70px block of dark bold text, mean lightness 25%, 320px from centre

These can balance, and no measurement will tell you exactly. But the model tells you which direction to move when it is wrong, which is the part you actually need.

The near-miss problem

The worst outcome is neither balanced nor deliberately unbalanced. A layout that is 5% off balance reads as a mistake, in exactly the way a 17px heading above 16px body text reads as a mistake. The threshold rule applies to composition as well as to type.

So commit. Either resolve the balance properly, or push the imbalance far enough that it is obviously a choice — an element hard against one edge with a large void opposite it, not an element drifting slightly right of centre.

This is the most common fault in first posters and first slides: everything is nearly centred, nearly balanced, nearly aligned. The fix is rarely subtle adjustment. It is usually to pick a direction and go much further.

Tension is balance you have spent on purpose

An unbalanced composition creates tension, which is useful when the subject wants it: urgency, movement, instability, a before-and-after where the before is meant to feel wrong. A figure placed hard against the right edge, looking off the page, with empty space behind, is unsettling, and that is a tool.

Two things keep tension from reading as an accident:

1. Magnitude. Far past the threshold, as above.
2. Consistency. If the composition is deliberately weighted left, everything else in the piece should agree — the type alignment, the crop, the direction of any implied motion. A layout weighted left with centred type is a layout in an argument with itself.

Checking it

Three free checks, in increasing order of reliability.

Flip it horizontally. Mirror the whole design. A composition you thought was balanced often collapses when mirrored, which tells you it was relying on the reading direction rather than on weight. This also catches the near-miss, because the near-miss moves to the other side and you notice the movement.

Blur it heavily and look at the blobs. From the first block. Balance is a property of the masses, and once the detail is gone you can see the masses.

Turn it upside down. The oldest trick in drawing instruction. Inverting the image suppresses your recognition of what the objects are and leaves you looking at shapes and weights. It is startlingly effective, and it takes one keystroke in any editor.

Balance is not centring

A final distinction, because the two get confused constantly. Centring is placing an object on an axis. Balance is a relationship between everything on both sides of that axis.

A centred headline over an off-centre photograph is centred and unbalanced. A left-aligned headline with a heavy image on the right can be perfectly balanced and contains nothing centred at all. Layouts that centre everything by default are not balanced; they have simply declined to make the decision, and the tell is that they usually feel inert without anybody being able to say why.

Symmetry is a decision. Centring everything is the absence of one, and readers can feel the difference even though they will describe it as the design being boring.