How many metres of string lights does your balcony need? Drape them and see

String lights are the quickest way to make a balcony feel like a small outdoor room after dark. They are also the thing people most often buy the wrong length of. Too short and the last swag ends in mid-air; too long and a tangle of spare cable lies under the chair. A tape measure and a bit of arithmetic fix both.

Three ways to hang them, three very different lengths

The same balcony needs a surprisingly different amount of cable depending on how you hang the lights:

• A straight run along the railing or wall uses exactly the width of the balcony. It is the shortest and tidiest option.
• Swags (loops that droop between hooks) look relaxed and festive, but the droop adds cable. Deeper loops and fewer hooks change the total more than most people expect.
• An overhead zigzag from the wall to the railing makes a glowing ceiling. It looks the most like a little room, and it needs several times the length of a straight run.

Try your own balcony below. Enter its width and depth in centimetres, pick a way of hanging, and watch the length change.

The rule of thumb behind the number

A straight run is just the width. A droop is where people get caught out. A hanging loop is close to a parabola, and its length is roughly the span plus 8 × droop² ÷ (3 × span). You do not need to calculate it, but the shape of the formula is useful: the extra cable grows with the square of the droop. Doubling the droop roughly quadruples the extra cable. A gentle 10 cm droop on a one metre span adds under 3 cm, while a 30 cm droop adds about 24 cm.

Fewer hooks means longer, deeper swags, so the total goes up even though the balcony has not changed. If you are short of cable, adding a hook usually costs nothing and saves more length than you would think.

For the overhead zigzag, the sum is simple: each pass is as long as the balcony is deep, and you add the gap each time the line turns around. A 400 cm wide, 150 cm deep balcony with a pass every 60 cm needs about 7 passes, so about 14 m of lit cable. That is why a canopy usually needs a long string, or two strings plugged end to end if the maker allows it.

When to break the rule: if you want a very relaxed, dense canopy, hang the passes closer together and accept the extra length. If you only want a soft glow, a straight run or a few swags along one side is plenty. Cosy is about warmth, not about covering every centimetre.

Lit length is not the length on the box

Most strings have a plain, unlit cable at the plug end, and sometimes a short one at the far end too. A pack that says 10 m may light only 8 or 9 m of it. Check the product page for the lit length, and for how long the lead cable is. If the listing gives only one number, assume the lit part is a little shorter. The tool above adds your distance to the socket separately, so the number it gives is the cable you need in total; check that the lit length plus the lead cable on the listing reaches it.

What to measure before you buy

• The width of the balcony along the railing or wall, and its depth from wall to railing, in centimetres.
• The height of your hooks or fixing points, so swags clear heads and furniture. Tall guests and parasols matter.
• The distance from the nearest outdoor-safe socket (or your battery or solar position) to the first hook. Measure along the route the cable will really take, not in a straight line.

Then use the tool and round up to the next length you can find. If you already own lights from a previous year, test them first: a string you have is a string you do not need to buy.

Power, weather and fixings

Choose lights marked for outdoor use. A rating such as IP44 means the casing is splash-proof, which is a sensible minimum for a covered balcony; a higher number such as IP65 copes better with open, rainy spots. Battery, USB and solar strings avoid the question of cable to a socket completely, though they need a good place to sit in the sun or a power bank in the dry. For a plug-in string, keep the plug and any connectors off the wet floor and out of puddles. If there is no outdoor socket and you would like one, that is a job for an electrician, not something to improvise.

In a rental, use clips, adhesive hooks, cable ties around the railing or clamps rather than drilling, and check your lease or building rules, because some blocks restrict what can be attached to a facade or railing. Balconies are windy places: tie the string at every hook rather than letting a long swag flap, and take lights down in a storm. Keep lights away from dry plants and fabric awnings, and switch them off when you go to bed.