Kalau celana ini terasa sesak di pinggang, saya mau tukar ukuran.

Kalau means if in this sentence.

So Kalau celana ini terasa sesak di pinggang, saya mau tukar ukuran means something like:

If these pants feel tight at the waist, I want to exchange for a different size.

A few notes:

• kalau is very common in everyday Indonesian
• it is less formal than jika or apabila
• in conversation, kalau is usually the most natural choice

Examples:

• Kalau hujan, saya naik taksi. = If it rains, I’ll take a taxi.
• Kalau kamu mau, kita bisa pergi sekarang. = If you want, we can go now.

In Indonesian, celana usually means pants / trousers as a single item of clothing.

So:

• celana ini = these pants / this pair of pants

Even though English uses pants as a plural-looking word, Indonesian celana is treated as a normal noun.

Examples:

• Celana ini mahal. = These pants are expensive.
• Saya beli celana baru. = I bought new pants.

If you really want to count items, you can use a classifier like sehelai in some contexts, but in normal speech people often just say:

• dua celana = two pairs of pants

Terasa means feels or can be felt.

In this sentence:

• celana ini terasa sesak = these pants feel tight

This is different from merasa, which usually means to feel in the sense of someone experiences a feeling.

Compare:

• Saya merasa lelah. = I feel tired.
• Celana ini terasa sesak. = These pants feel tight.

Why?

• merasa usually has a human experiencer as the subject
• terasa is often used when something seems / feels a certain way

So for clothing, food, temperature, texture, etc., terasa is very natural:

• Bajunya terasa kasar. = The clothes feel rough.
• Ruangan ini terasa dingin. = This room feels cold.

Not exactly.

In this sentence, sesak means tight or too tight / constricting.

So:

• terasa sesak di pinggang = feels tight at the waist

Difference:

• sesak = tight, cramped, constricting; often about discomfort
• sempit = narrow, too small in width/space

For clothes, both may appear in related situations, but sesak emphasizes the uncomfortable tightness.

Examples:

• Baju ini sesak di dada. = This shirt is tight in the chest.
• Jalan ini sempit. = This road is narrow.
• Kamar itu sempit. = That room is small/narrow.

Also, sesak is used in other contexts too:

• sesak napas = shortness of breath
• ruangan sesak = a crowded/cramped room

Di pinggang means at the waist or around the waist.

• di is the basic preposition for location: in / at / on
• pinggang = waist

So:

• sesak di pinggang = tight at the waist

This is a very normal way to describe where clothing feels tight.

Other examples:

• Kemeja ini sempit di bahu. = This shirt is tight in the shoulders.
• Rok ini longgar di pinggang. = This skirt is loose at the waist.
• Sepatu ini sakit di kaki. = These shoes hurt my feet.

Yes, mau literally means want to, but in everyday Indonesian it often also means:

• would like to
• intend to
• am going to

In this sentence:

• saya mau tukar ukuran = I want to exchange for another size

In a shop, this sounds natural and common.

Compare:

• Saya mau bayar. = I want to pay / I’m ready to pay.
• Saya mau coba yang ini. = I want to try this one.
• Saya mau tukar ukuran. = I want to exchange sizes.

If you want to sound a bit more formal or polite, you could also say:

• Saya ingin tukar ukuran.
• Saya ingin menukar ukuran.

Yes, it is natural in casual spoken Indonesian, especially in a shopping context.

Tukar means exchange / swap.

So tukar ukuran literally looks like exchange size, but the real meaning is:

• exchange this item for a different size

The thing being exchanged is understood from context: celana ini.

In fuller Indonesian, you might also hear:

• Saya mau tukar celana ini ke ukuran lain.
• Saya mau menukar celana ini dengan ukuran yang lebih besar.
• Bisa tukar ke ukuran lain?

So the original sentence is conversational and slightly shortened, which is very common in real life.

Because Indonesian does not use articles like English a, an, and the.

Instead, definiteness is often understood from context, or shown with words like:

• ini = this
• itu = that

So:

• celana ini = these pants / this pair of pants

Without ini, celana could mean pants in a more general or indefinite way.

Compare:

• Saya beli celana. = I bought pants / a pair of pants.
• Saya beli celana ini. = I bought these pants / this pair.

Yes, you could, and it would still be understandable.

Compare:

• Kalau celana ini terasa sesak di pinggang...
• Kalau celana ini sesak di pinggang...

The version with terasa sounds a bit more like:

• if these pants feel tight at the waist

The version without terasa is more direct:

• if these pants are tight at the waist

Both are natural, but terasa adds the sense of physical sensation.

Yes. Indonesian allows some flexibility.

Original:

• Kalau celana ini terasa sesak di pinggang, saya mau tukar ukuran.

You could also say:

• Saya mau tukar ukuran kalau celana ini terasa sesak di pinggang.

Both mean basically the same thing.

The version with kalau first sounds a little more like setting up the condition first:

• If this happens, then I want to do this.

That is very common in Indonesian.

Yes, it is understandable and fairly natural, but it is somewhat direct and conversational.

If you want to sound more polite, you could say:

• Kalau celana ini terasa sesak di pinggang, saya ingin tukar ukuran.
• Kalau celana ini terasa sesak di pinggang, bisa tukar ukuran?
• Maaf, kalau celana ini terasa sesak di pinggang, apakah saya bisa tukar ukuran?

Useful shopping phrases:

• Bisa tukar ukuran? = Can I exchange for a different size?
• Ada ukuran yang lebih besar? = Is there a bigger size?
• Ada ukuran yang lebih kecil? = Is there a smaller size?

Good question. In everyday Indonesian, especially in speech, bare verbs are very common.

So:

• saya mau tukar ukuran sounds natural in conversation

But in a fuller or more formal style, you might use:

• menukar

Examples:

• Saya mau tukar ukuran. = casual, natural speech
• Saya ingin menukar ukuran. = more complete/formal

This happens a lot in Indonesian. Spoken language often drops prefixes when the meaning is already clear.

Compare:

• Saya mau beli ini. instead of Saya mau membeli ini.
• Saya mau ganti baju. instead of Saya mau mengganti baju.

Sometimes, yes, but the meaning shifts a little.

• tukar ukuran = exchange for a different size
• ganti ukuran = change the size

In a store, if you want to return one item and get another size, tukar ukuran is clearer.

Examples:

• Saya mau tukar ukuran. = I want to exchange for another size.
• Bisa ganti ke ukuran yang lebih besar? = Can I change to a bigger size?

In real conversation, people may use both, but tukar strongly suggests an exchange.

You can add the specific size you want.

Examples:

• Saya mau tukar ke ukuran yang lebih besar. = I want to exchange for a bigger size.
• Saya mau tukar ke ukuran yang lebih kecil. = I want to exchange for a smaller size.
• Ada ukuran satu nomor lebih besar? = Is there a size one step bigger?
• Ada size lebih besar? = Is there a bigger size?

In shops, you will often hear a mix of Indonesian and English, especially with size:

• Ada size M?
• Saya mau tukar size L.

That kind of code-mixing is very common in everyday Indonesian.