不X不知道，一X吓一跳: you only find out by looking
【六59】口语格式 / fixed patterns
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
不V不知道，一V吓一跳: the same verb comes back twice. The first half is a condensed condition: 不查不知道, "if you don't check, you don't know". The second is a 一……就…… with its 就 dropped: 一查吓一跳, "the moment you check, what a shock". Together: "you have no idea until you look closely".
It takes mostly verbs of checking (查, 看, 算, 称, 量, 问, 比, 试). The surprise is usually bad (a number too high, a price, a delay), but it can be good: 不吃不知道，一吃吓一跳，居然这么好吃. 吓一跳 can also give way to another ending: 不做不知道，一做才发现真不会.
Register: spoken, lively, among friends or in a vivid story; a report would rather say 经检查发现…. Do not confuse it with 一……就…… (一下课就回家), the basic structure it is a shortened version of.
Examples
不称不知道，一称吓一跳，我这个月竟然重了四公斤。
Bù chēng bù zhīdào, yì chēng xià yí tiào, wǒ zhège yuè jìngrán zhòng le sì gōngjīn.I had no idea until I weighed myself, and then what a shock: I've put on four kilos this month!
不算不知道，一算吓一跳，这个月光外卖就花了一千多块。
Bú suàn bù zhīdào, yí suàn xià yí tiào, zhège yuè guāng wàimài jiù huā le yì qiān duō kuài.I only found out when I added it up: takeaway alone cost me over a thousand yuan this month.
不问不知道，一问吓一跳，这么小的房子，租金竟然要八千块。
Bú wèn bù zhīdào, yí wèn xià yí tiào, zhème xiǎo de fángzi, zūjīn jìngrán yào bā qiān kuài.I was stunned when I asked: for such a small flat, the rent is eight thousand yuan.
不查不知道，一查吓一跳，家里的旧电线早就坏了。
Bù chá bù zhīdào, yì chá xià yí tiào, jiāli de jiù diànxiàn zǎo jiù huài le.We had no idea until we checked, and what a shock: the old wiring in the house had been shot for ages.
这道题看起来简单，不做不知道，一做才发现真不会。
Zhè dào tí kàn qǐlái jiǎndān, bú zuò bù zhīdào, yí zuò cái fāxiàn zhēn bú huì.This problem looks easy, but you only find out when you try it that you really can't do it.
Common mistakes
✗ 没看不知道，一看吓一跳。 → ✓ 不看不知道，一看吓一跳 (the formula is fixed with 不, even when the story is in the past).
✗ 不看不知道，一查吓一跳。 → ✓ 不看不知道，一看吓一跳 (both halves repeat the same verb).