DIY :: How to make a 3D 21 (or any number!)

Last updated on 19 June 2020

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It’s surprisingly easy to make this 3D 21!

It was my sister’s 21st birthday not too long ago, and while we had a mostly family party (she lives in another city, and doesn’t have many friends here in Auckland, so she had a party with her friends, and then it was mostly family and a few others here for a fun evening), I still went a little mad with the decorations. One of the things that I actually made quite last minute – in the few days prior that is – was a giant 3D 21, and it was actually really easy and simple to do. While you can follow this tutorial for whatever number (or letter!) you’re creating, for the purposes of this post, I’ll be talking about ‘2’ and ‘1’.

What you need to make a 3D 21

• Cardboard (cardboard boxes work very well for this!)
• Tape (masking, duct, sellotape or something similar)
• A pencil
• A ruler
• Scissors or craft knife
• Your choice of decorating items  – in this case, tissue paper and wrapping paper.

How to make it

Decide how big you want your numbers in height and draw off a line at the top and bottom so you know inside what boundaries to draw in. Draw the general shape of a ‘2’ onto your cardboard, making sure you’ve within those boundaries. Once you have the general shape, you need to make sure that all parts of the ‘2’ are the same width.

To do this, start with one edge (e.g. the outside of the ‘2’), and grab your ruler. Decide how thick you want your numbers. I did mine at 13cm. From the outside edge of your ‘2’, mark a bunch of dots or lines 13m in, so you can adjust your drawing to a uniform size around the entire ‘2’. Straighten up your lines if need be, 13cm in, using the marks or lines you just drew. Do the exact same thing with the ‘1’. The ‘1’ is a lot easier, because you can use a ruler for the entire thing, and don’t have to free hand the curves.

You should now have a ‘2’ and a ‘1’ cut out. Place each one on top of more cardboard, and trace around it. This will ensure that your two sides of the complete 3D number are exactly the same. Cut them out. You now have two ‘2’s and two ‘1’s.

Now decide how thick you want your numbers; I decided on 10cm. Cut out a heap of cardboard strips at 10cm wide. Make sure you have some longer ones so you can do the long straight edges of the ‘1’ without having ‘seems’ – for want of a better word –  in the cardboard. Line up your sides with the edge of a section of your number (e.g.: the base line of the ‘1’) and cut the side cardboard strip to fit it lengthwise. Next, grab your tape and tape it flat, edge to edge, with the number. To help you see what we’re doing, you’re taping it flat, but if you pull up the side piece so it’s at a 90 degree angle to the number, you’ll see what they will be the sides for the eventual 3D-ness of the number. The 10cm side widths actually become 10cm heights for a while, while you’re working flat. This is what we’re going to do in a moment.

Continue cutting and taping your side pieces to your numbers, following the edges of the number. While I suggest taping all the side pieces to the number’s edges before sticking the side pieces to each other, if you find it easier to do, you can stick one piece, and another, and then prop those two pieces up to a 90 degree angle and tape them together. Whatever you find easier, go with that.  When it comes to the curves of the ‘2’, cut lots of little pieces of cardboard (remembering that they all need to be 10cm wide). While this is more time consuming and fiddly, it will mean you get a nice curve, like this:

Once your sides are all taped to the number and to each other, you have what I can only described as a mouse maze. It’s like a little tunnel with walls. You can, of course, leave it like this and simply prop your numbers up against a wall so you don’t see that there’s no back, but if you want to continue…take your other number, and tape it securely to the top of your nearly-3D number. And you’re done!

You may find that the edges don’t quite match up, especially on the curves of the ‘2’, but when you decorate it, you won’t be able to see them. Decorate in anyway you’d like. If you want to create the effect I had, read on!

Tissue Paper Scrunches

To make sure that there was going to be a similar/pretty colour behind my tissue paper scrunches in case I ended up having little gaps, I covered my numbers in wrapping paper, making sure it was very well stuck to the numbers. I used tape and the present-wrapping-method to cover them, but you can use glue to stick it down.

Take your tissue paper and cut out heaps of squares/rectangles. The smaller the are, the more time it will take and the more textured your finished product will look. The bigger the easier. Up to you. I cut my squares at about 10cm.

Using your thumb and index finger, grab the middle of the circle and pinch the paper, and twist it. You’re creating a sort of flowery scrunchie sort of shape. Grab your hot glue gun, and go nuts with sticking it your letters. You don’t have to do the back if you don’t want to (I didn’t), but if people are going to be seeing the other side, then it might pay to.

Other decorating ideas:

• Paint them (alternatively, carefully drop glow stick insides over it)
• Pierce little holes in them and leave a gap in the taping. Fill the numbers with fairy lights. The numbers   will hide the lights, but they will shine through the holes.
• Cover in cool wrapping paper or wallpaper
• Buy fake flowers and stick them all over the numbers
• Turn them into piñatas (add a fringe effect in layers with crepe or tissue paper. Fill with candy)

And there we have it! I hope everything made sense. I do apologise for not taking more photos of the first stage. I only thought a blog tutorial might be fun after I had finished creating the cardboard numbers. But I think it’s pretty easy to figure out. If you do have any questions, though, don’t hesitate to ask!

If you end up making these, do tag me in a photo (@Anjali_Kay on Twitter or @anjalikay on Instagram) or send me a picture. I’d love to see them!