If you actually call your father or mother's father 'Grandfather' then it seems quite formal. Most common would be Grandpa or Grandad.
Eg "We are all going to visit grandad this weekend."
自分の[父親](https://eikaiwa.dmm.com/uknow/questions/81725/)、または[母親](https://eikaiwa.dmm.com/uknow/questions/52602/)の父親のことを、 'Grandfather' と言い、かなりフォーマルな言葉です。Grandpa もしくは Grandadというのが最も一般的でしょう。
例：
"We are all going to visit grandad this weekend."
今週、みんなでおじいちゃんのところへ行きます。
Grandfather is a formal way of saying 'father of my parent'.
A casual way of saying it would be either 'Grandad'. In American English, it is spelled 'Granddad', but in British English it is spelled 'Grandad'.
A very easy way to say it is 'Grampa' or even 'Gramps'. These two are commonly used by very young children.
everyday speech: grandfather, grandpa, granddad, etc.
"paternal" and "maternal" are good words to use if it's important whether the grandparent is your mother's parent or your father's parent. These words often appear if you are talking or reading about a person's ancestry or DNA. Americans whose families came from another country (that is, almost everybody) can use it if they are trying to talk precisely about their family origins. However, paternal/maternal grandmother/grandfather are formal-sounding words, so, for example, you wouldn't use it if you are introducing your grandparents to someone. In normal conversation, you might just say, for example, "Here is my grandfather", even though it's not immediately clear which grandfather he is.
「paternal（父方の）」と「maternal（母方の）」は、祖父母が「母方」なのか「父方」なのかを伝えたいときに便利な言葉です。祖先やDNAについて話したり、読んだりする時によく出てきます。他の国からきた家族をもつアメリカ人（ほとんどみんなですが）は、正確に家族の起源を話す場合に、これを使います。
ただ「paternal/maternal grandmother/grandfather（父方/母方の祖母/祖父）」にはフォーマルな響きがあって、例えば、誰かに祖父母を紹介するときには使いません。
普通の会話では：
"Here is my grandfather"
こちらが私の祖父です。
どちらの祖父かはっきりしませんが、このように言います。
Grandfather' is often the most formal way of referring to the father of one's parent.
We can use 'grandad' and 'grandpa' to be more casual. Many people have their own variety of ways to refer to their grandparents such as 'gramps' and 'pops', amongst others.
Grandfather is more formal, but "Grandpa" and "granddad" are more casual ways of referring to your father's father. For example; you can say:
-I am visiting my paternal granddad for Christmas this year because I haven't seen him in two years.
-My maternal grandpa is the best. He always gets me the best gifts for my birthday.
Grandfather はよりフォーマルですが、 "Grandpa" や "granddad"は、祖父を表すよりカジュアルな言い方です。
-I am visiting my paternal granddad for Christmas this year because I haven't seen him in two years.
二年間会っていないので、今年のクリスマスに父方のおじいちゃんのところへ行きます。
-My maternal grandpa is the best. He always gets me the best gifts for my birthday.
私の母方のおじいちゃんは最高だ。彼はいつも私の誕生日に最高のプレゼントをくれる。
Depeding on what country you come from would decide what term you use to describe your mother or fathers father
In the UK 'grandfather' or 'grandad' would be more common to say
Here the most commonly used words to describe the father of one's parent would be to say the above "Grandfather" "Grandad" or "Grandpa".
Here are some examples -
"I'm going to see my grandfather today."
"Myself and my granddad are going to the shopping centre today".
おじいちゃんのことを"Grandfather" "Grandad" "Grandpa"と言うのが一般的です。
例文：
"I'm going to see my grandfather today."
（今日おじいちゃんに会う予定です。）
"Myself and my granddad are going to the shopping centre today"
（私とおじいちゃんは今日ショッピングセンターに買い物に行く予定です。）
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