MathCal Developer Interview: Building Arithmetic Skill by Hand
Hiroki Obara · Developer
MathCal is an arithmetic app for elementary school children who write their answers by hand on the screen. Version 1.0.0 was released on the App Store in July 2025. It runs on iPhone and iPad, and the app is available in 16 languages.
The Column Arithmetic Drill arrived in February 2026, the bead-based Montessori lessons in April, and Today’s Drill in May. Version 1.5.0 in July dropped “StudyX” from the name and brought the app to 22 drills and 59 lessons. I asked hiroki, who made it, about how it started and how they think about it now. The answers are kept as close to the original Japanese as translation allows.
A handwriting drill from day one
Q. When you started MathCal, what screen did you have in mind?
From the start, the concept was an arithmetic drill you answer by handwriting. There are plenty of arithmetic drill apps without handwriting, but almost none where you write the answer. I built it on a scientific fact I learned while making BrainDojo: if you do not write by hand, the brain does not really get working.
Q. How different is today’s app from what you first set out to make?
At first it was a simple arithmetic drill. Bit by bit, it has come to cover the whole elementary school curriculum. I added lessons and keep developing it. It will keep growing until this app alone can cover the elementary school math curriculum.
Q. Where does the name come from?
From Math & Calculation. It is MathCal outside Japan and マスカル算数 (MathCal Sansū) in Japan because each name is easier to remember in its market.
Turning boring math into play
Q. What kind of child were you when it came to math?
Personally, I was good at both math and arithmetic. Even so, I was bored by school lessons and paper drills. I thought it would be good if it became interactive play, like a video game, so I made it.
Q. Whose point of view do you take when you build an educational app for children?
The child’s. The users are children, so I build it with children first.
Moving your hand gets the brain working
Q. Why handwriting instead of number buttons?
Because moving your hand gets the brain working. Tapping away at a screen does not do much for the brain.
Q. In the column drill, Check answer cannot be pressed until every box has been read, and each recognised digit is shown small at the edge of its box. How did you arrive at that?
I actually watch my daughter use it, and I fix the places where there are problems.
Q. In the column division drill, only the quotient and remainder boxes are checked, and the working can be written freely on the grid. Why not grade the working?
Because it would take away the comfort.
Q. The column drill does not support landscape. Why portrait only?
Column arithmetic is written downwards, so the screen has to be tall.
Q. The settings include recognising 5 written in two strokes, a left-handed answer screen and a choice of division sign. What led to those?
I added them after watching people actually use the app.
Q. The school grade is worked out from the birth year and month using the Japanese school year. For children in other countries, the grade boundaries may not match.
Basically it follows the Japanese curriculum. But I have translated it into many languages and localised it as far as I can, so it can be used anywhere in the world.
Q. Today’s Drill is the same all day, however many times you open the app. Why one drill per day rather than a random one each time?
Because practising once a day is enough.
Understanding and practice, like two wheels
Q. Calculators are everywhere. What is the point of a child being able to calculate quickly by hand?
Reading, writing and arithmetic are basic literacy, and they are the base for many of the brain’s abilities. So even if you can use a calculator, I think arithmetic skill should be trained. It supports the ability to think.
Q. Drills that race the clock and lessons where you move beads to understand how things work seem to point in opposite directions. Why put both in one app?
I think it is good to turn understanding and practice together, like two wheels.
Q. What drew you to Montessori education? When you move real materials onto a screen, what is lost and what can only a screen do?
That it is child-centred. Thinking with your hands is also unique to it. There is the idea that children deepen their understanding intuitively while playing with their hands, and I designed this app so that idea can be put into practice.
Because it is a digital app, it cannot fully reproduce the feel in the hand or the three-dimensional texture. That is where I feel the real thing is still needed. Setting problems interactively and answering them is something only an app can do. I hope people will use the two together well.
No ads
Q. In an app a child uses alone, what have you decided you will never do?
Show ads. As a parent too, I care about being able to let a child use it alone without worry.
Q. The parental check asks for the current year in four digits rather than a math problem. Why not a math problem in a math app?
If the child is good at arithmetic, a math problem would make the parental check pointless.
When my daughter got interested in column arithmetic
Q. After the October 2025 update, there is a gap until the column drill arrived in February 2026. Why did you start again with column arithmetic?
Because my daughter started to get interested in column arithmetic.
Q. What was hardest about the column drill?
Reproducing, with handwriting recognition, the feel of doing column arithmetic by hand on paper. The hardest part was designing the interface so you can tell intuitively what to write where.
From the people who use it
Q. Are there any ways the app has been used, or reactions to it, that stayed with you?
I often get negative feedback about the accuracy of handwriting recognition. Everyone’s handwriting is different, so it cannot be perfect. On top of that, it has to be recognised at high speed. I feel there is still a lot of room for technical improvement.
Q. The app is in 16 languages, but most ratings come from Japan. Do you notice differences between children abroad and in Japan?
My discovery was that in Europe and North America, children do not do much arithmetic practice. So most users are in Asia. It made me feel that Japan is one of the Asian countries.
Q. The three daily picks are free and the rest is a one-time purchase of 1,500 yen in Japan. Why a one-time purchase, and how did you think about the price?
Since it is used in Asia, I thought a one-time purchase would be better. As for the price, there is no marking to do, the amount of practice is on another level, and the records stay with you. Compared with paper workbooks and study guides, I think it is good value.
On to middle and high school math
Q. Where do you want to go next: adding more, or deepening what is there?
I want to cover Japan’s elementary school math curriculum completely. Right now it is something a parent or another adult uses alongside the child while explaining things. That is fine in its own way, as a communication tool, but I think the content still has room to improve. I want to build up both quality and quantity. After that, the plan is to widen the scope to middle school math and then high school math.
Arithmetic as a path
Q. When children who used MathCal grow up, even if they do not remember the app, what do you hope stays with them?
Trained arithmetic skill. Reading, writing and arithmetic are the foundation of intellectual activity, so arithmetic matters as a cognitive ability. I think it will stay important however far AI advances. On the other hand, if people become less and less intellectual from here on, and turn into living things that are not intellectual at all, I think that would be sad.
Social media, chat tools and short videos are at their height, and I feel that a childhood spent absorbed in them is a waste and not very healthy. I really worry about the future of children growing up in a society that creates dependence and addiction. As a humble app developer, I want to keep quietly making “something that helps children become smarter.”
Q. Has making this app changed how you think about math or learning? Is there a question you have not answered for yourself yet?
Busy work like drills is not recommended in Montessori education. But I feel that the “love of arithmetic” in Asia, Japan included, is something Europe and North America cannot understand. Doing arithmetic at high speed can be called play, like a mind sport, and it can also be called a path to mastery, like a martial art. If a child takes it up of their own accord, I think it is a good habit for the brain too. Being fast at arithmetic is probably more useful at work than being slow, and I think leaving everything to computers is risky.
There is a claim that paper sticks in your head better than digital, and I feel that may be true. The texture of paper really is good. Even when I take notes, I want to write on paper rather than on the slippery screen of an iPad. I feel paper and digital should be used together well, taking the best of each.
About MathCal
MathCal has 22 arithmetic drills, from one-digit sums through column arithmetic, fractions and decimals to ratios and speed, and 59 Montessori lessons where children move beads to learn the four operations, fractions and shapes. Every answer is handwritten. Today’s Drill, Today’s Challenge and Today’s Lesson on the home screen are free every day. Choosing any drill or lesson from the full lists requires a one-time unlock. There are no ads.
How to start is in Getting Started with MathCal, and how to teach column arithmetic is in Teaching Column Addition and Subtraction at Home. For details about the app, see the MathCal page.
