SumNinja

SumNinja is a free browser game for building mental math. A learner picks a skill and a session length, starting at two minutes, and answers quick questions at their own pace. Correct answers gradually open up more challenge, and mistakes bring the difficulty back within reach, so a child who is struggling is not left stuck on questions that are too hard. The short session gives a clear finish line, which makes it easy to fit in as a daily warm-up rather than a full lesson.

Practice covers arithmetic, fractions and simple algebra, and a learner can narrow in on one skill such as a single times table. The site tracks accuracy and speed, along with a seven-day view, streaks and achievements, so a child or parent can see how practice is adding up and decide what to work on next.

What lifts it above a plain drill game is the Learn section. It holds 27 short lessons on practical strategies, grouped into integers, fractions, decimals, algebra, functions and vectors. The integer lessons, for example, cover adding faster with a make-10 strategy, solving harder calculations by rounding to an easier number, and turning difficult multiplications into easier ones. There is also a math glossary for any word that needs explaining. The later function and vector lessons go beyond what most elementary students need, so the site can stay useful into middle school.

SumNinja is free, has no ads, needs no sign-up to play and can be played offline. It is a supplement for speed and confidence with number facts and mental strategies, not a full math curriculum, so it pairs best with whatever program your family already uses. It was built by an independent maker, Kim Døfler, and the site is available in many languages, which may help families raising multilingual learners.