How to Use DUPLO In Your LEGO MOCs
Today’s guest article comes from Evan Crouch, an AFOL out of Colorado, a historical MOC builder, contributor to Brethren of the Brick Seas and the Builder Improvement Initiative, and founder of Rocky Mountain Minifigs. You can find his work on Instagram.
What Is DUPLO?
Have you ever wondered how you can possibly create a large mountain or landscape using your limited brick collection? Among other interesting methods, one great technique for building big is to integrate DUPLO into your LEGO builds.
DUPLO is a line of large building bricks introduced by LEGO in 1969. A 2x4 DUPLO brick is twice as large as a LEGO system brick, meaning it is twice as long, twice as wide, and twice as tall. In practice, this means a single DUPLO brick can fill the space of eight LEGO System bricks!
Interestingly, LEGO also briefly produced a line of extra-large bricks called QUATRO, which are four times as large as regular LEGO bricks and twice as large as DUPLO. Unfortunately, QUATRO bricks were phased out in 2006, though you can still find them on BrickLink.
While DUPLO was originally introduced as a larger building block safe for toddlers and small children, it holds an exciting secret—DUPLO bricks can connect securely with regular LEGO bricks!
Connecting LEGO to DUPLO
The bottom tube on some LEGO system bricks allows them to connect securely to the open studs on DUPLO.
Many 2x and bigger LEGO System bricks will fit on top of DUPLO, but unfortunately not all of them will. Specifically, some of the older, larger bricks lack underside tubes or have conflicting underside support ribs that get in the way. It’s worth noting that many rectangular DUPLO bricks will also securely connect to LEGO baseplates—unfortunately, round DUPLO will collide with LEGO studs and won’t sit flush with the baseplate.
In 2002, LEGO updated the DUPLO brick mold to reduce the height of the underside support ribs. This change to the mold allowed DUPLO bricks that were produced in 2002 and later to connect to QUATRO bricks in the same way as DUPLO bricks can connect to LEGO system bricks.
Using DUPLO as Supports or Filler Brick
One of the main characteristics of DUPLO is its large size. With large DUPLO bricks in the core of a MOC build, you’ll need fewer system bricks to supersize your model. This has the added benefit of making building quicker, stronger, and cheaper than using many other scaffolding/filler brick techniques.
In many recent builds, I have used DUPLO to quickly create terrain. This can be accomplished by connecting DUPLO bricks to a baseplate, building up fairly close to your desired height, and then capping off the top DUPLO brick or plate with a System brick.
In this example, I created a rough terrain profile using light bluish-gray 2x4 DUPLO bricks. Next, I topped those DUPLO bricks with 2x4 LEGO System bricks and then added some side details with LEGO plates and slopes. For the interior of the modules, I capped the DUPLO bricks and plates with larger green 8x8 and 8x16 LEGO bricks. Because the DUPLO portion of this build was extremely straightforward, these five modules only took a few hours to put together.
Similarly, when I was creating terrain sloped down to a beach, I again started with DUPLO bricks and capped the terrain with LEGO System bricks and plates. This process was pretty quick—it took about 30 minutes.
Both of these examples are from my recent 40-baseplate Colonial New Brickford build. Because of the use of DUPLO, I was able to quickly make the majority of this massive build in a few weeks leading up to a Spring 2025 LEGO convention.
DUPLO bricks are often crammed together in a solid manner when being used as supports, and this highlights one of the downsides of using DUPLO as terrain-building filler: they can be heavy compared to other LEGO supports or pillars! A few ways to get around the weight issue are to 1) use “hollow” DUPLO elements like window/door frames or arches as these will still securely support bricks but some of their volume is taken up by air, or 2) to space out your support DUPLO into pillars and then connect them together using larger DUPLO plates or System bricks.
Featuring DUPLO In a Visible Way
Because DUPLO bricks are made by LEGO out of similar plastic and coloring as smaller System elements, the color match is spot on. This means that a medium nougat DUPLO brick will blend in seamlessly next to a medium nougat LEGO System brick.
Large DUPLO slopes in light or dark bluish-gray can be integrated into your LEGO rockwork. In the late 1990s and early 2000s, LEGO also produced part 31071, which is a large DUPLO rockface element. This can be a fun easter egg to include in your mountainside!
Can you spot the large DUPLO rockface piece in this Rocky Mountain National Park MOC? Perhaps that piece should be nicknamed “BPRP” for Big Pretty Rock Piece.
Once you’re aware of the connection opportunities between DUPLO and LEGO, the options are limitless. Danthefan has used a DUPLO cone to top off a medieval tower roof. Dreamweb built a LEGO pirate ship on a DUPLO pirate ship base—it floats in water! How might you include some DUPLO parts in your next build?
DUPLO-only Builds
DUPLO can also be used with modern SNOT building techniques to create fun DUPLO-only MOCs.
As an aside, while LEGO strove to make DUPLO figures difficult to take apart for safety reasons, they can be disassembled and rebuilt in different configurations. Using the blunt end of a 7/64” drill bit, you can pop out the hip pin to then take apart the figures. This allows you to make any number of different DUPLO figure combinations!
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Where to Buy DUPLO for Cheap
If you were to look at DUPLO bricks on LEGO’s Pick and Brick site, you’d likely be dissuaded from giving them a try! A single 2x4 DUPLO Brick will run you $1.05 each there. This means 100 2x4 DUPLO bricks would be $105… plus tax! While ordering from LEGO will give you the benefit of free shipping and large available quantities, it can be pretty expensive.
Instead, I recommend checking out BrickLink, Facebook Marketplace, and local thrift stores or yard sales in your search for DUPLO. On BrickLink, you can often find the same 2x4 bricks that cost $1.05 each from LEGO for $0.05 each! While you will have to pay for shipping, at the time of writing you can often get 100 2x4 bricks for less than $20 shipped. On Facebook Marketplace and at local thrift stores and yard sales, you can also find DUPLO bricks for cheap - families often part with their DUPLO collections when children graduate onto smaller LEGO system bricks, meaning you can acquire their DUPLO inexpensively if you time it right! If only they knew that DUPLO and LEGO System bricks connect together!
Using DUPLO in MOCs
So the next time you’re planning a massive landscape or a towering cliffside, don’t overlook the humble DUPLO brick. Whether you’re using it for structural support, visual flair, or even a floating pirate ship, DUPLO offers AFOLs a surprising amount of versatility. It might be designed for toddlers, but with a little creativity, it can help elevate your most ambitious LEGO builds.
Do you build with DUPLO? Let us know in the comments below!
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