Softer than you think. The IRF rules put a roundnet ball between 2000 and 3500 pascals, which is 0.3 to 0.5 PSI. Inflate it until it holds a round shape and still gives clearly when you press hard with a thumb. If it feels like a tennis ball, let air out.
Here is the awkward part. That pressure is so low that most pumps and almost every gauge in a household cannot read it. There is no number on a dial to aim for. You do this by hand, and once you know what right feels like you never forget it.
Written by Jakub Popluhar, roundnet coach since 2019, 1,000+ coaching hours, former national coach of Belgium.
Hold the ball in one hand and press hard with your thumb. It should dent visibly and spring straight back. Not squash flat. Not resist you like a rock. Somewhere in between, and much closer to the soft end than anyone expects on their first try.
Then drop it on the net. A correctly inflated ball on a correctly tensioned net comes back fast and predictably. If it thuds and dies, one of the two is wrong, and the honest way to find out which is to test them separately. The net has its own drop test, which is on the setup page.
"Why not just give me a PSI number, Jakub?" I did, it is in the first paragraph. The problem is that 0.3 PSI sits below the useful range of the gauge on a normal pump. You would be reading zero and guessing anyway.
Three numbers. All from the International Roundnet Federation ruleset:
There is a fourth line that people skip over, and it is the one that matters most for you at home: the ball has to allow internal pressure adjustment. A roundnet ball is built to be tuned. That is not an accident and it is not a manufacturing shortcut, it is a design decision, and it is what lets you make the same ball right for a six year old and right for a tournament.
About the size, since the numbers do not help much: 330 mm around puts it somewhere between a grapefruit and a small melon, and 85 g makes it far lighter than it looks. That combination is why it moves the way it does.
Any ball pump with a needle does the job. The same needle that goes into a football or a basketball, and a bike pump with a needle adapter works fine too. Sports shops sell the needles for the price of a coffee. Most households with a child already own one.
One warning, because it goes wrong in the other direction to what you would expect. At this pressure you are not fighting to get air in, you are trying not to put too much in. A couple of strokes, check by hand, a couple more. Overinflating is the standard first mistake and it makes the ball painful to receive.
Wet the needle before it goes in. Dry needles tear the valve, and a torn valve is a dead ball.
A hard ball is a fast ball. It leaves the net quicker, gives you less time, punishes every small error in your first touch, and stings the forearms of anyone who is still learning to receive. A soft ball does the reverse: slower off the net, more time, more rallies, and it forgives a bad contact.
Neither is correct in the abstract. They are correct for different people. Put two identical teams on a hard ball and a soft ball and you get two different sports, which is exactly why the tournament rules pin the range down instead of leaving it to whoever brought the set.
This is the single cheapest adjustment in the whole game. It costs nothing, takes ten seconds, and it changes more about how an evening in the garden feels than any technique tip I could give you.
It might be. The fix is not a different ball. Let air out of the one you have.
Take it to the soft end of the range, or a little under it if nobody is keeping score. The ball slows down, the hands stop hurting, and rallies suddenly last four or five touches instead of one. Children do not quit because a game is hard, they quit because nothing ever comes back over. Longer rallies fix that in an afternoon.
Same trick for adults who have never played a ball sport, by the way. Nobody has to know.
Two reasons. Both boring. Balls lose air slowly, all of them, and cold air takes up less room. A ball that was perfect in August will feel flat on a cold morning and needs a couple of strokes before you start.
Balls are consumables. They go soft, the surface wears smooth, and one day they stop holding their shape. Competitive players get through them and think nothing of it. Keep one spare inflated in the bag and you will never lose an evening to it.
Between 2000 and 3500 pascals, which is 0.3 to 0.5 PSI. Too low for a normal gauge, so you judge it by hand: round shape, clear give under a hard thumb press.
Any needle pump works, the football and basketball kind. Go slowly, because at this pressure the danger is putting too much in, not too little. Wet the needle first.
330 mm around and 85 g, per the IRF rules, with small tolerances on both. Bigger than a grapefruit, lighter than it looks.
Let air out. The ball is built to be adjusted, and a softer ball is slower, kinder on the hands and produces the longer rallies that keep children playing.
Lost pressure, cold weather, or both. Add a little air and test by hand. If it will not hold air between sessions, the valve or the bladder is finished.
For a garden game, whatever bounces off the net is fine. For tournament play the rules require an IRF approved ball, and the weight, circumference and pressure ranges are all specified.
Press your thumb into the ball right now. If it barely moves, you have found the reason the game felt harder than it should.