index.js

/** UI element heights which used in message height calculation */
textPaddingVertical: number = 7;
textPaddingHorizontal: number = 10;
timeHeight: number = 12;
timeTopMargin: number = 5;
messageContainerVerticalMargin: number = 7;
dateSeperatorVerticalMargin: number = 20;
dateSeperatorHeight: number = 15;
imageHeight: number = 170;
imageBottomMargin: number = 10;
avatarHeight: number = 45;
windowWidth: number = Dimensions.get("window").width;
/** END */

calculateMessageHeight(message: Message, index: number): number {
    const textHeight = message.messageHeight;
    let messageHeight = textHeight;
    //If time seperator is shown
    if (
        message &&
        !isSameDay(
            message.created,
            getSafe(
                () => this.props.data.getDataForIndex(index + 1).created,
                null
            )
        )
    ) {
        messageHeight =
            messageHeight +
            this.dateSeperatorHeight +
            this.dateSeperatorVerticalMargin * 2;
    }

    if (message.type === MessageType.IMAGE) {
        messageHeight =
            messageHeight + this.imageHeight + this.imageBottomMargin;
    }

    messageHeight =
        messageHeight +
        (this.timeHeight + this.timeTopMargin) +
        this.textPaddingVertical * 2 +
        this.messageContainerVerticalMargin * 2;

    if (messageHeight < this.avatarHeight) {
        messageHeight = this.avatarHeight;
    }

    return messageHeight;
}

layoutProvider = new LayoutProvider(
    (index) =>
        this.props.data.getDataForIndex(index)?.type ?? MessageType.MESSAGE,
    (type, dim, index) => {
        const message = this.props.data.getDataForIndex(index);
        dim.width = this.windowWidth - 15 * 2;
        dim.height = this.calculateMessageHeight(message, index);
    }
);
    
    
/* Here snippet how I calculate text height */
import rnTextSize, { TSFontSpecs } from "react-native-text-size";
import { Dimensions } from "react-native";

const fontSpecs: TSFontSpecs = {
    fontFamily: "Poppins-Regular",
    fontSize: 13,
};
const textPaddingHorizontal: number = 10;
const maxTextWidth: number =
    Dimensions.get("window").width * 0.65 - textPaddingHorizontal * 2;

const textHeight = rnTextSize.flatHeights({
                ...fontSpecs,
                width: maxTextWidth,
                text: [message.text],
            });