The most common budgeting mistake isn't spending too much — it's picking a round number first ("$50 a day!") and only later checking whether that number is real for the destination. Build it the other way around.
Start with lodging, since it's the most fixed cost
Look up 5–6 actual hostel or budget guesthouse listings for your destination and take the median price, not the cheapest outlier — the cheapest listing is usually cheap for a reason. That median is your lodging line.
Add food using a per-meal estimate, not a daily guess
Estimate breakfast, lunch, and dinner separately using real local prices (street food, markets, and set-menu lunch spots, which are almost always cheaper than tourist-zone restaurants), then add 15% for snacks and drinks you'll inevitably buy.
Build in a "surprise" category — it's not optional
Every trip has an unplanned cost: a missed bus, a better tour than expected, a rainy day that turns into a museum day. Budget 10–15% of your daily total for this rather than letting it blow the whole budget when it happens, because it will happen.
Takeaway: Recalculate every 3–5 days against actual spending, not just at the trip's start — a budget built once and never checked against reality isn't a budget, it's a guess.